Vision

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]

The electroretinogram b-wave is a prominent positive-going component of many light-evoked electroretinography traces, associated strongly with the activity of ON retinal bipolar cells. It is an extracellular tissue-level signal, rather than the intracellular membrane potential of one photoreceptor or one bipolar cell. Its appearance and amplitude depend on the stimulus and recording conditions.^[ref]

The assignment was tested by perturbing retinal pathways. In amphibian preparations, APB suppressed ON bipolar responses and removed the b-wave; treatments suppressing OFF-related responses removed the light-offset d-wave while preserving the b-wave. Recording potassium changes along with field responses helped link the electrical component to a specific functional pathway.^[ref]

A later molecular perturbation supplied independent evidence. Mice lacking TRPM1 retained a normal rod a-wave but lacked the b-wave, while direct bipolar-cell recordings supported disruption of postsynaptic ON signaling. A signal from the early photoreceptor stage can therefore persist when the later response is lost.^[ref]

For an illustrative trace, a preceding trough at $-80\mu \mathrm{V}$ and a positive peak at $120\mu \mathrm{V}$ are separated by $200\mu \mathrm{V}$. That trough-to-peak difference differs from the peak's $120\mu \mathrm{V}$ displacement above a zero baseline. Always state the amplitude convention; these invented values are not diagnostic limits or data from either study.

The visual cycle is the set of biochemical transformations that regenerates the light-sensitive retinal supply of visual pigments after illumination. In the classical rod pathway, a photon absorbed by rhodopsin converts its bound 11-cis-retinal toward all-trans-retinal. Subsequent processing involving the photoreceptor and the retinal pigment epithelium returns retinal to a form that can rebuild light-sensitive pigment. This recycling connects a rapid light-triggered molecular event to slower recovery of sensitivity.^[ref]

A useful experiment interrupted this supply rather than removing the receptor itself. T. M. Redmond and collaborators disrupted RPE65 in mice and measured retinal compounds and visual responses. The animals lacked normal rhodopsin and rod electroretinography responses even though rod opsin remained; all-trans retinyl esters accumulated, while 11-cis retinoids were absent. These results established that RPE65 is necessary for the normal production of 11-cis vitamin A derivatives in this pathway.^[ref]

The contrast supplies an illuminating example of biochemical dependence: a cell may retain a receptor protein yet lose the response because it cannot regenerate the small molecule required by that receptor. The visual cycle is the chemical recycling pathway; electroretinography measures an electrical consequence of its success or failure. The knockout establishes necessity in the tested pathway and conditions, rather than proving that every photoreceptor follows an identical regeneration route.

Spatial and temporal sampling of light by a finite receptor array. In a regular one-dimensional point-sampling model with spacing $d$, frequencies $f$ and $f+k/d$ give identical sinusoidal samples because the phase difference at sample $n$ is $2\pi kn$. Restricting a real signal below the Nyquist frequency $1/(2d)$ removes that basic ambiguity. Real retinal mosaics are irregular, two-dimensional and optically filtered.

A photoreceptor class important for low-light vision. Greater sensitivity and downstream pooling support detection under different conditions from cone-dominated vision. When only one spectral receptor class supplies usable information, the principle of univariance prevents independent wavelength–intensity identification.

A light-absorbing pigment involved in a receptor response. Spectral sensitivity depends on pigment absorption together with factors such as optical filtering and pigment density; a pigment name does not specify a perceived color.

Conversion of absorbed light into a photoreceptor response. In vertebrate rods and cone cells, activation of visual pigment changes a signaling cascade and light-sensitive membrane current, typically hyperpolarizing the cell and changing transmitter release. A receptor voltage, ganglion-cell spikes and a reported color are distinct measurement levels.

Light-driven reduction or change in available light-absorbing pigment. Its consequences should be distinguished from gain adjustments observed without a comparable loss of pigment.

A receptor's absorption-dependent response does not independently label the wavelength of an absorbed photon. Different combinations of spectral composition and intensity can yield the same receptor quantum catch. Comparing spectrally distinct receptor classes can supply information unavailable from one alone. In the displayed model, $N$ is a photon distribution and $a$ effective absorptance; an energy spectrum would require the corresponding photon-energy conversion.

A vertebrate retinal photoreceptor class important in photopic vision and colour discrimination.

A cone class relatively sensitive at shorter wavelengths. Its signal is compared with other cone signals, and its sparse retinal sampling differs from the combined L/M mosaic.

A cone class with an intermediate relative spectral sensitivity. Its broad overlap with L-cone sensitivity makes comparisons informative; a response does not label a stimulus green.

A cone class with spectral sensitivity relatively shifted toward longer wavelengths compared with M and S classes. Its broad response is not a detector for a unique red sensation.

Dependence of a receptor or sensory response on wavelength. Define whether the measurement concerns a pigment, receptor, circuit or behaviour.