Sensory system

Structures and processes that acquire and transform information about external or internal conditions, including receptors, transduction and downstream neural processing.

A sensory cell or nerve ending that transduces a physical or chemical stimulus. This meaning is broader than a single biochemical receptor protein.

A muscle spindle is a specialized stretch-sensitive receptor within a skeletal muscle. It contains small intrafusal muscle fibres with sensory endings that report changes in their mechanical state. This supplies proprioception: information about the body's own position and movement, even without seeing the limb.

The receptor is adjustable. Motor input to the intrafusal muscle fibres changes tension within the spindle and can alter the response of its sensory neurons. In a cat soleus experiment, researchers electrically stimulated an individual motor fibre supplying a muscle spindle, then applied a controlled ramp stretch while recording the sensory output. The prior stimulation changed the subsequent burst of impulses.^[ref]

The same imposed movement can therefore produce different sensory signals depending on the spindle's previous activation. A muscle spindle is an actively regulated sensor, rather than a passive ruler reading muscle length independently of neural state.

An intrafusal muscle fibre is a specialized skeletal muscle fibre within a muscle spindle. Its mechanical state is monitored by endings of sensory neurons, contributing to proprioception, and motor input adjusts that state. The ordinary force-producing extrafusal muscle fibres lie outside the muscle spindle. Intrafusal contraction therefore helps regulate a sensory apparatus as well as changing local tension.

In a cat soleus preparation, researchers repetitively stimulated a single motor fibre supplying a muscle spindle, then imposed a ramp stretch while recording a primary sensory ending. Prior motor stimulation produced an extra burst of impulses at the start of the stretch. A subsequent stretch abolished this after-effect, whereas without that stretch it could persist for more than a minute.^[ref]

Two stretches with the same imposed movement can thus produce different sensory signals because the intrafusal muscle fibres have different recent activation histories. The authors proposed persistent interactions between actin and myosin as a mechanism; that proposal should be distinguished from the directly recorded impulses.^[ref]

A receptor responsive to pressure-related stretch. Cardiovascular baroreceptor signals provide one route by which the heartbeat can influence central processing.

Reduction or absence of input in a sensory modality. Onset, residual sensitivity, cause and duration matter when comparing compensation; a diagnostic category does not imply identical sensory experience.

A category of severe auditory impairment or absent hearing. Onset, residual hearing, language experience and task demands matter in interpreting compensatory performance.

A category of severe visual impairment or absent vision, defined differently by different functional or clinical criteria. Congenital, early and later-onset cases can differ in residual vision and experience; none should be presumed to possess a universal enhancement of other senses.

Experimental or environmental restriction of sensory input. Temporary deprivation can reveal state-dependent changes, but it is not automatically equivalent to established sensory loss with a different developmental history.

Sensory processing of chemical stimuli. Gustation and olfaction provide distinct receptor and delivery systems; their contributions to flavor should be separated from oral mechanical and thermal inputs.

Sensory processing of the body, including mechanical, thermal and other bodily inputs. Touch is one component; not every bodily sensation is a tactile spatial signal.

Perception of mechanical contact or deformation through somatosensory receptors and processing. Discrimination depends on stimulus geometry, body site, exploratory action and task, not solely receptor density.

Perception of object properties through touch and exploratory action, often combining cutaneous and proprioceptive information. Active exploration changes which information becomes available.

A category of sensory input and processing, such as vision, hearing or gustation. Receptor type, pathway and experienced quality provide related classifications, but should not be assumed to define identical boundaries.

Sensing dissolved chemical stimuli through taste receptors. In ordinary language, taste can also refer to an integrated food flavor; experimental work must distinguish this broader usage from gustatory transduction.

Detection and processing of acoustic stimulation. Signal detectability depends on the receiver and background noise.

Auditory feedback is information obtained by hearing the sound produced by one's own action. During vocal control, it can be compared with an intended or learned sound, allowing later production to be adjusted. Hearing another individual's demonstration supplies an example to learn; hearing the consequences of one's own production supplies feedback. The two experiences have different causal roles.

A controlled birdsong experiment manipulated that distinction. Adult Bengalese finches wore custom headphones; a cage microphone fed sound-processing hardware that shifted the pitch of their own songs before playback. The birds changed their produced pitch in the compensatory direction, and pitch returned toward baseline when the perturbation was removed. Stable adult song therefore remained subject to active correction under these conditions.^[ref]

For an original numerical illustration, a playback system that multiplies frequency by $2^{1/12}$ turns a $500$-hertz component into about $529.7$ hertz. To cancel that multiplicative shift exactly, production would need to be about $500/2^{1/12}=471.9$ hertz. These values illustrate the direction of an ideal compensation, not a claim that the birds achieved complete correction.

Altered auditory feedback can thus probe a control mechanism without simply disabling hearing. Timing, pitch and loudness are distinct perturbations, and a response to one manipulation does not establish identical control rules for every species or vocal behavior.

Audiometry measures aspects of hearing under specified stimulus and response conditions. A tone-detection threshold depends on frequency, presentation route and the task. It is a psychophysical measurement rather than a complete description of every auditory ability.

Measurement of the mechanical response of the tympanic membrane and middle ear as ear-canal pressure changes.

The cochlear nerve fibres carrying auditory information from hair cells toward the brain.

Perception of a speech sequence as containing a segment whose acoustic information has been replaced by another sound. The classic contrast with a silent gap shows the importance of the replacement sound as plausible masking evidence. The perceived continuity does not mean that the missing waveform has been reconstructed from the ear signal alone.^[ref]

Processes that organize a mixture of sound into perceptual streams and auditory objects. They must balance grouping evidence against the need to separate overlapping sources and update representations as events change.

A direction-dependent acoustic transfer function from an external source to one ear, including effects of the head, pinna and torso under stated measurement conditions. Individual geometry changes the spectral cues. A pair of such functions predicts binaural signals for a known source; inferring direction from unknown sounds is a harder inverse problem.

A difference in sound level between the ears, often influenced by the head's direction- and frequency-dependent acoustic shadow. It complements interaural time differences, rather than serving as an exact independent direction label in every environment.

The difference in arrival time or corresponding temporal structure of sound at the two ears. In an ideal far-field point-receiver model with separation $d$, $\Delta t=d\sin \theta /c$ follows from the path difference divided by sound speed. Head diffraction, multiple sources and reverberation complicate this approximation; one delay need not identify a unique direction.

Sound qualities supporting distinctions between sources beyond overall level, duration and pitch. Spectral shape, temporal envelopes and their changes contribute; no single scalar need describe every timbral difference.

A perceptual attribute allowing sounds to be ordered as relatively high or low. It is related to, but not identical with, acoustic frequency or the location of the largest component in a frequency spectrum. Complex periodic signals require consideration of temporal and spectral structure.

An orderly relation between preferred acoustic frequency and anatomical position in a sensory structure. Cochlear mechanical tonotopy originates in graded mechanics, while later tonotopic neural organization involves additional processing. A frequency map is not itself a complete sound-object representation.

The fluid-filled inner-ear structure whose graded mechanics and sensory cells transform sound-driven vibration into neural signals. Distributed frequency selectivity, active feedback and temporal coding contribute distinct information; a cochlear place response alone does not identify the physical source of a sound.

The sensory apparatus supported by the basilar membrane and containing cochlear hair cells. Their mechanotransduction helps convert vibration into auditory signals.

Active biological feedback contributing level-dependent gain and frequency tuning to cochlear mechanics. It can enhance weak near-characteristic-frequency stimulation more than intense or off-frequency stimulation. A perturbation of the whole preparation must be interpreted carefully because it can alter both mechanical and neural transduction processes.

A sensory cell whose mechanically sensitive bundle supports mechanotransduction. The cochlear inner and outer classes have different dominant roles in afferent signaling and mechanical feedback.

A cochlear hair-cell class contributing mechanical feedback that increases weak-sound sensitivity and sharpens frequency selectivity. Its feedback-dependent effects are part of the cochlear amplifier.

A cochlear hair-cell class providing most afferent input to the auditory nerve. Mechanical stimulation changes receptor activity and transmitter release; downstream auditory nerve fibers carry action potentials.

A mechanically graded structure supporting frequency-dependent traveling-wave responses in the cochlea. High-frequency responses peak nearer the base and low-frequency responses nearer the apex. Its living response is modified by the cochlear amplifier, so passive mechanical measurements do not capture the entire operating system.

The tympanic membrane and ossicular coupling system that transfers airborne vibration toward the fluid-filled inner ear. Its mechanical transformation helps address an acoustic impedance mismatch; it is not an ideal, lossless, frequency-independent amplifier.

The tensor tympani muscle is a middle-ear muscle attaching to the malleus. Its contraction alters the mechanics of the tympanic membrane and ossicular chain. Its innervation is associated with the mandibular division of the trigeminal nerve, rather than the facial supply to stapedius.

The tubotympanic recess is an embryonic extension of the first pharyngeal pouch contributing to the auditory tube and middle-ear cavity. Its endodermal origin differs from the ectodermal contribution to the external ear passage.

The tympanic cavity is the air-containing middle-ear space housing the auditory ossicles. It communicates with the nasopharynx through the auditory tube. The cavity, its lining and the structures suspended within it have different mechanical and developmental roles.

The Eustachian tube connects the middle ear to the nasopharynx, enabling pressure equalization, mucosal clearance and protection from nasopharyngeal contents.

Interference with detection or discrimination of a sound by other sounds. Signal frequency, amplitude, timing and receiver processing can affect masking.

Interference involving uncertainty, source confusion or selection beyond a simple account of overlapping acoustic energy. Demonstrating it requires suitable controls for audibility and peripheral overlap; merely finding a difficult listening task is insufficient.

Reduced target detectability associated with overlap between target and interfering acoustic energy in the listener's representation. It is distinguished conceptually from informational masking, although both can occur in one task and the distinction does not identify one anatomical site automatically.

Sensory information about bodily position, movement and force, contributing to coordination and relations between seen and felt movement.

Quantitative study of the relationship between controlled physical stimuli and sensory performance or reports.

An approximate regime in which a threshold increment is proportional to background intensity, so $\Delta I/I$ is roughly constant. It is a task- and range-dependent empirical relation, not a universal law at photon-limited levels or under all adaptation states.

A transient change in electrical conductance of the skin, often used as an index of autonomic arousal. Electrode recordings provide a physiological response measure, not a direct reading of threat beliefs or diagnostic status.

A judgment of which of two events occurred first or second. Changing the response rule, modality reliability or attention can alter measured order without implying a corresponding change in physical stimulus timing.

A judgment of whether two events occurred together. Its response curve across onset differences depends on timing noise and acceptance criteria; it need not equal the psychometric function of temporal order.

Let internal timing difference be $D=\delta +b+\epsilon$ with $\epsilon \sim N(0,s^2)$, and report simultaneous when $-h<D<h$. Standardizing the two interval bounds gives $P(\mathrm{same})=\Phi ((h-\delta -b)/s)-\Phi ((-h-\delta -b)/s)$. For positive $h,s$ and symmetric noise, this function peaks at $\delta =-b$. Its width reflects both noise and acceptance bounds, so it cannot identify a sensory window without additional assumptions.

An offset at which events are judged most simultaneous under a specified measurement procedure. A fitted peak in simultaneity reports need not coincide with the equal-response point of an order task or with equal neural onset latencies.

Measurement of gaze direction or fixations using a specified recording method and calibration procedure. Prespecified regions of interest, usable-data criteria and time windows are needed to connect gaze records to a developmental hypothesis.

An experimental method using the duration of looking as a dependent measure. A difference can reflect novelty, familiarity, surprise or processing demands, so psychological interpretation depends on the design and controls rather than on duration alone.

Presenting outcomes predicted to be congruent or incongruent with a proposed expectation and comparing looking responses. In false belief research, manipulating an agent’s visual access while controlling actual object location helps distinguish belief-based predictions from direct reality tracking. Longer looking is an indirect measure requiring alternative explanations to be tested.^[ref]

A central attention cue precedes a side cue and an auditory stimulus. The infant’s orientation controls or indexes the duration of exposure, permitting comparison of responses to stimulus classes. Masking sound for adult observers helps reduce inadvertent cueing.^[ref]

Assigning a perceptual contribution to an available response scale because a more appropriate scale is absent or unclear. An odor contribution rated only on a sweetness scale could therefore affect the score without establishing a separate gustatory sensation. Providing multiple scales and independent discrimination tests can help test this possible explanation; their results should not be presumed in advance.

A task requiring selection between two specified alternatives, such as which of two intervals contained a signal. For balanced alternatives and no usable information, expected correct performance is one half. Its decision assumptions and possible response biases must still be specified; forced choice does not remove every source of bias.

A framework separating stimulus discriminability from decision criterion in noisy tasks. Response bias and sensory sensitivity need not change together.

The ability to discriminate stimulus classes, conceptually distinct from the decision criterion determining which response is selected. A higher frequency of fear judgments can reflect a criterion shift without better discrimination.

Under an equal-variance Gaussian evidence model, $d^{\prime }$ is separation of signal and noise means in units of their common standard deviation. A yes/no task gives $d^{\prime }={\Phi }^{-1}(H)-{\Phi }^{-1}(F)$, where $H$ and $F$ are hit and false-alarm probabilities. This formula assumes the model; it does not define every possible sensory-discrimination measure.

Compare independent Gaussian samples $R_s,R_n$ with means ${\mu }_s,{\mu }_n$ and common variance ${\sigma }^2$. Choosing the larger is correct when $R_s-R_n>0$. The difference has mean ${\mu }_s-{\mu }_n$ and variance $2{\sigma }^2$, so standardization proves the displayed relation. Correlated samples, unequal variances or a different response rule need a different calculation.

A rule or boundary converting internal evidence into a response, such as reporting a signal when $R>c$. Changing the criterion can change hits and false alarms without changing the separation of the underlying evidence distributions. A measured response threshold is therefore not automatically a pure measure of sensory sensitivity.

A function relating a stimulus variable to measured response probability or discrimination performance.

An empirical psychometric function whose core $F$ ranges from zero to one, lower asymptote is $\gamma$, and upper asymptote is $1-\lambda$. If $F(\alpha )=1/2$, then $p(\alpha )=\gamma +(1-\gamma -\lambda )/2$. A location parameter is consequently not a task-independent threshold. For a two-choice observer randomly guessing on fraction $r$ of otherwise perfect easy trials, the upper error asymptote is $r/2$, not $r$.

Let internal evidence be $R=\mu (x)+\epsilon$ with zero-mean Gaussian noise of standard deviation $\sigma$, and respond yes when $R>c$. Taking a distribution tail gives $1-\Phi ((c-\mu )/\sigma )$, which equals the displayed expression by symmetry. Its derivative is $\varphi ((\mu -c)/\sigma ){\mu }^{\prime }(x)/\sigma$. Thus sensory gain, variability and criterion all influence the observed relation, and a sigmoid in one stimulus coordinate need not remain symmetric in another.

A stimulus value corresponding to a specified performance criterion in a specified task. It is a summary of a psychometric function, not a claim that every trial below that value has no sensory response. State the response definition, chance level, lapse convention and performance criterion.

Detection and processing of volatile chemical stimuli.

An olfactory sensory neuron is a neuron that detects odorant molecules and sends an electrical signal toward the brain. Receptor-bearing sensory endings sample chemicals at the nasal surface, while the axon carries the resulting output toward the olfactory bulb. Detection and interpretation are therefore distributed across peripheral cells and central circuits.

Genetically labelled receptor-expressing neurons allowed investigators to trace where their axons converged in the olfactory bulb. Receptor-swap experiments further tested how receptor identity contributes to that projection pattern. ^[ref] This connects molecular recognition to circuit organization without implying that a single active neuron uniquely identifies every odour: an odour can recruit a pattern across receptor classes.

Olfaction involving food volatiles reaching the nasal cavity from the mouth through the nasopharynx. It contributes to flavor and often to oral attribution. Changing this route can alter transport and peripheral stimulation as well as object context.

Olfaction involving odorants entering the nasal cavity through the nostrils. It commonly samples external sources; its relation to external object attribution is an empirical question rather than a definition of every possible experience.

A vertebrate brain structure receiving projections from olfactory sensory neurons.

A local site of converging sensory axons and interactions with postsynaptic cells. Molecular identity and other developmental mechanisms contribute to targeting.

A receptor involved in detecting odorants. Receptor identity is one component of the mapping from molecules to neural and behavioural responses.

Sensing based on light, including detection, discrimination and spatial processing.

Dark adaptation is recovery of visual sensitivity after exposure changes from brighter surroundings to darkness or much dimmer conditions. Operationally, a fixed test stimulus becomes detectable at a lower intensity as adaptation proceeds. This is more than the increase in light capture caused by dilation of the pupil: changes in retinal sensitivity and visual cycle chemistry also matter.

The direction of a threshold plot can be misleading. In an invented example, detection threshold falls from 1,000 to 1 arbitrary intensity unit under otherwise matched testing conditions. Sensitivity defined as reciprocal threshold has increased 1,000-fold; the threshold has fallen by three logarithmic units. A descending curve therefore represents improving sensitivity.

Selig Hecht and collaborators measured how preceding light exposure changed subsequent human dark adaptation. Stronger pre-exposure produced an initial cone-dominated phase followed by a delayed rod phase in their experiments, while lower pre-exposures did not show the same two-phase pattern. The exposure history is thus part of the measurement, not an irrelevant preparation detail.^[ref]

The visual cycle provides a biochemical connection. RPE65-deficient mice lacked normal rod pigment and rod responses despite retained opsin, showing that producing the receptor chain alone cannot replace the chemical supply needed for functional rhodopsin.^[ref]

The knockout is not a normal human recovery curve. Psychophysical thresholds, electrical electroretinography responses and pigment chemistry measure different levels of the system. Comparisons of adaptation curves should specify stimulus properties and preceding exposure rather than assuming a universal recovery time.

Identifying an object from visual information. Perceptual integration, access to knowledge and retrieval of a name are distinguishable requirements.

Loss of vision in half of the visual field. A field defect can coexist with neglect but does not by itself explain all attentional or representational omissions.

The region of space available to vision under specified viewing conditions. Field testing and attention testing assess different potential reasons for missing a target.

Visual pathways involving occipital and parietal systems that contribute to spatial processing and visually guided action. This functional emphasis does not imply independence from attention or other visual pathways.

A computation mapping visual information into coordinates and commands appropriate for action, taking account of variables such as target, gaze and limb position.

Regularities in images produced by objects, illumination and viewing geometry in a specified environment. They can provide useful priors, but a correlation in one dataset or rendering family is not a universal inverse rule.

The distribution of pixel luminance values without their spatial arrangement. Moments such as mean, variance and skewness describe aspects of that distribution, while losing relationships among locations.

Permuting pixel locations preserves the multiset of luminance values, hence its histogram and every sum ${\sum }_{i}f(I_i)$, including raw moments. It need not preserve spatial correlations, edges or relations between highlights and shading. This proves nonuniqueness of an image from its histogram; a particular perceptual consequence requires experiment.

Vision under conditions in which rods and cone cells can both contribute. Describing all dim-light performance as either purely scotopic or purely photopic can miss their interactions.

A tendency for object decisions to follow local texture rather than global shape under cue conflict. Human–model comparisons on matched conflict images can reveal different strategies despite similar ordinary classification accuracy. Altered training distributions can change the bias without changing the network architecture.^[ref]

Recognition that preserves object or face identity despite changes in viewpoint. The aim is tolerance together with selectivity: responding identically to every object would be invariant but useless for distinguishing identity. Tolerance may be partial and differ across processing stages.

A set of visual cortical pathways contributing to object and face recognition. Hierarchical neural network models use growing receptive fields, feature combination and learned transformation tolerance to explain some response properties.

Visual computation in which activity feeds back within or between processing stages, allowing later representations to depend on earlier internal states. Time-resolved object-recognition measurements constrain this contribution more strongly than a static accuracy score, although a delay alone does not identify one circuit.^[ref]

Searching a visual display or scene for a target defined by a task. Response time, accuracy, eye movements and stopping behavior provide different constraints. Candidate set size, similarity, visibility, prevalence and scene context all matter.

Changes in search behavior and misses when targets become more or less frequent. A controlled artificial baggage-search study found more misses at low target prevalence, together with altered absent-decision times. The phenomenon motivates modeling decision criteria and stopping in addition to feature processing; laboratory rates are not automatically professional field rates.^[ref]

A model family in which initial visual information and task goals prioritize candidate objects for selective processing. Selection is not necessarily a random scan, and serial selection can coexist with overlapping recognition processes.

A 2021 modeling development by Jeremy M. Wolfe, combining feature-based guidance, selection history, reward and scene information with selection, recognition and quitting processes. It models serial deployment with overlapping processing rather than assuming that only one complete operation occurs at a time. Model timings are hypotheses tested by simulation, not direct anatomical measurements.^[ref]

Assume $N$ candidate items are examined once each, each costs time $\tau$, nonsearch processing costs $b$, and a uniformly positioned target ends search when encountered. The target rank has mean $N^{-1}{\sum }_{k=1}^{N}k=(N+1)/2$, proving the present-trial expression. Absent trials inspect all $N$ items. The resulting 2:1 slope ratio is a benchmark under these assumptions, not a unique diagnostic of a serial mechanism.

Search in which the target is distinguished by a particular combination of features that individually occur among distractors. Guidance and grouping can make such search more efficient than exhaustive random item-by-item inspection.

Search in which an individual target feature distinguishes it from distractors. Efficient performance can produce a shallow response-time/set-size relation, but that relation alone does not prove absence of attention or a strictly unlimited parallel mechanism.

A rapid eye movement between fixation positions. In a memory-guided saccade task, the endpoint provides a behavioral report of a previously presented location; sensory retention and prospective movement coding can therefore covary.

Visual sensitivity to ultraviolet wavelengths. Its relevance to a colour comparison depends on the receiver’s actual receptors and the illumination.

A light-sensitive receptor protein associated with a chromophore. Sequence, expression, chromophore and neural processing all contribute to visual performance.

A sensory cell converting light into a biological response.

Electroretinography records electrical responses generated by the retina when it is stimulated with light. In full-field electroretinography, diffuse flashes stimulate a broad retinal area, and an electrode at or near the eye records a potential difference relative to a recording reference. The trace is a summed response of retinal tissue, rather than a recording of one photoreceptor or a direct report of perceived brightness.^[ref]

Dark-adapted and light-adapted recordings help distinguish response systems under controlled conditions. Standardization matters because flash strength, adaptation and electrode configuration change the trace. A simple experimental comparison uses the same flash and recording arrangement before and after an intervention: differences then become evidence about which retinal processes contribute, subject to the intervention's selectivity.^[ref]

The positive b-wave illustrates how component interpretation is tested. Richard A. Stockton and Malcolm M. Slaughter measured retinal field responses together with extracellular potassium changes in amphibian preparations. A drug that suppressed ON retinal bipolar cell responses also removed the b-wave; treatments suppressing OFF-pathway responses removed the light-offset d-wave while preserving the b-wave. This supported a central role for ON retinal bipolar cell activity in generating the b-wave, rather than treating all deflections as direct rod voltages.^[ref]

An independent example is the RPE65 knockout: disrupted visual cycle chemistry caused loss of normal rod responses on electroretinography, connecting pigment regeneration to a measurable tissue-level signal.^[ref]