Perceptual Processes ch 3

Spatial Vision: From Spots to Stripes

• Spatial vision is a low-level processing
• Sensory signals travel from photoreceptors, through vertical pathways to the optic nerve, and then retinal ganglion cells.
• Signals then travel through the lateral geniculate nucleus of the thalamus.
• Finally, they reach the striate visual cortex(primary visual cortex) in the occipital lobe.
• Different types of cells in the retina, like P cells (parvocellular) and M cells (magnocellular), transmit information differently to the brain (parvocellular-ventral, magnocellular-dorsal)

Which Way Do Eye Signals Go?

• Visual signals travel from photoreceptors to the optic nerve via retinal ganglion cells.
• Pathways go through the lateral geniculate nucleus of the thalamus.
• Lastly, signals are sent to the visual cortex, specifically in the occipital lobe of the brain.

From Retina to Brain

• Signals from P cells and M cells follow distinct pathways.
• Parvocellular stream, for identifying "what"
• Magnocellular stream, for "where" or "how"

Building Perception from Sensory Pieces

• Ganglion cells respond to points of light in their receptive fields.
• Visual cortex cells respond to lines, edges, bars, and stripes, inferred from ganglion cell inputs.
• Visual cortex is further subdivided into smaller regions specializing in processing orientation, width, color, etc, in extra striate cortex.
• Information from these areas is combined in secondary visual cortical regions creating a coherent view of the world.

A Few Terms

• Contrast refers to the luminance difference between an object and its background.
• Acuity is the smallest spatial detail resolvable at 100% contrast.
• Visual angle is the angle subtended by an object at the retina.
• Grating cycle is a pair of one light bar and one dark bar forming a complete grating image.

Vision Contrast Test System

• Each row displays a particular visual grating frequency (stripes).
• For each circle in the row, indicate whether the grating shows left, right, up, or blank.

Acuity

• Eye doctors use 20/20 vision (e.g., normal vision) to evaluate and characterize visual acuity at a standard distance.
• A lower denominator for 20/XX test indicates poorer acuity.
• Smallest visual angle/visual cycle that can be detected denotes better eyesight.

Vision Grating Test

• The test is used to determine visual acuity, evaluating the ability to distinguish stripes/gratings at varying frequencies.

Cycle Example

• A grating cycle is the repeating pair of a light bar and a dark bar in a visual grating.

Acuity (Cont)

• Herman Snellen invented a method to specify visual acuity in 1862
• Vision tests often include letters (such as the letter E) to measure acuity

So Why Stripes?

• Our visual system is made of cells that detect light spots more efficiently than when light is seen as a whole.
• Center-surround cells are organized for better identification and effective detection of edges of objects.

Center-Surround Cells Revisited

• Center-surround cells' receptive fields respond differently to stimuli in the center versus the surround.
• Center stimulation often has a stronger response compared to the surround, resulting in a boundary appearing darker or lighter depending on the cell type and stimulation.

Acuity and Stripes

• Resolutional acuity is the finest high-contrast detail that can be distinguished (identified).
• The ability to distinguish detail depends on the retinal's photoreceptor spacing.
• Sine-wave gratings demonstrate how visual acuity works.

Acuity and Stripes (Cont)

• Visual system "sees" spots across grating; each circle represents the size of the cell's receptive field. Smaller receptive fields in visual acuity result in perceiving striped patterns.

Receptive Field Sizes

• Cones in the fovea are densely packed, thus generating smaller receptive fields, compared to rods and cones in the periphery.
• Horizontal and vertical receptive field densities are not the same, with a higher density across the horizontal axis (better acuity left/right) than the vertical axis (better acuity above/below).

Back to Stripes Again

• Spatial frequency is the number of grating cycles per degree of visual angle.
• Schade studied the relationship between spatial frequency and contrast sensitivity.

Spatial Frequency

• Contrast sensitivity is optimized around medium spatial frequencies, better than at lower or higher frequencies.

Contrast Sensitivity Function

• Contrast sensitivity is consistent with highest contrast at the top and lowest at the bottom.

Contrast Sensitivity (Cont)

• Sine-wave grating frequency remains the same across the chart from top to bottom, with an increase in frequency from left to right.

Contrast Sensitivity Function (Cont)

• Individual contrast sensitivity curves vary.
• Myopia can reduce contrast sensitivity in severe cases.
• Contrast sensitivity tends to decrease with age.
• Ambient light, eye adaptation, and other factors adjust the contrast sensitivity curve.

So, Why Sine Wave Gratings?

• Stripes are essentially boundaries that the visual system identifies.
• Sine-wave gratings are rarely found in nature, but the visual system is tuned to process them since these patterns are common.

So, Why Sine Wave Gratings? (Cont)

• Visual system breaks down images into a vast collection of sine-wave gratings with each a specific spatial frequency.
• This method (Fourier analysis) is effective for processing images in our visual perception, similar to sound processing by the system.

Horizontal Lines--Vertical Lines--Light Bands--Dark Bands--Edges

• Different lines (horizontal, vertical, light bands, dark bands, edges) all generate individualized sine-wave gratings.
• Combination of those generates a complex visual pattern perceived to be as complex as speech or music in the brains process for decoding of the image.

Tuning a Ganglion Cell

• Different cells in the on-center ganglion cells have different tuning responses based on receptive field size.
• The signal weakens when the frequency of the grating is either too low or too high, or not appropriately in tune with the cells' receptive field tuning.

It's Just a Phase

• In addition to being tuned to certain frequencies and orientations, cells must also be in phase (synchronized) with the sinusoidal grating pattern to produce a response.

If You have Photosensitive Epilepsy...

• Individuals with photosensitive epilepsy and light-triggered migraines may want to avoid light stimuli characterized by stripes or other patterns to avoid trigger reactions.

Feeling a Little Funky?

• Stripes can cause discomfort.
• Sensory perception differences in people with photosensitive epilepsy and migraines are related to heightened sensitivities, causing higher levels of cell firing, which can be influenced by gamma oscillations.
• Visual illusions can be caused by this increased cell firing.

Feeling a Little Funky? (Cont)

• Visual stimulation by stripes leads to cell synchronization at 30-90 Hz, and this generates sensations of dizziness, nausea, and headaches in some individuals, even causing seizures.

The Lateral Geniculate Nucleus of the Thalamus

• The lateral geniculate nucleus (LGN) is organized into layers.

The LGN is Like an Onion...It Has Layers

• The LGN is divided into magnocellular layers (processing motion signals), parvocellular layers (processing fine detail signals) and koniocellular layers (interconnecting the other layers)

The Visual Field is Divided in Half

• The visual field is halved and each half is seen by a half of each of the retinas.
• Left LGN receives visual information from the left half of each retina, and conversely the right LGN receives visual information from the right half of each retina.

The LGN is Like an Onion... (Cont)

• Layers of the LGN process and pass on information from each eye, in the following pattern: Layers 1, 3, and 6 transmit from the contralateral retina, and layers 2, 4, and 5 transmit information from the ipsilateral retina.

Striate Visual Cortex

• Striate cortex (primary visual cortex, area 17, or V1) is a significant part of the visual processing pathways.
• A substantial conversion of visual information takes place in this region
• Receptive fields in the retina and LGN are transformed into elongated "strip" receptive fields in the cortex.
• The striate cortex has roughly 200 million cells, a much greater amount than the lateral geniculate nucleus (LGN).

Striate Visual Cortex (Cont)

• Striate cortex is comprised of six layers, similar to the LGN.
• Fibers from the LGN project primarily to layer 4C.
• Magnocellular axons send projections to upper 4C.
• Parvocellular axons project to lower 4C.
• Projections from other cortical areas are processed in other layers and are sent back out for further processing.

Cortical Mapping and Magnification

• Visual field regions correlate to equivalent areas in the striate cortex.
• The fovea is assigned a larger processing area in the cortex relative to the periphery (magnification).

Topography of Visual Cortex

• Cortical layout was originally learned via animal models and human lesion studies—lesions were produced intentionally to understand the function of different parts of the cortex.
• MRI and fMRI are used to map the human visual cortex today.
• MRI measures brain structure and fMRI measures brain activity using blood oxygen level-dependent signals.

Hypercolumns and Blobs

• Hypercolumns are approximately 1 mm in size; they contain sets of columns for every possible visual orientation, with one set for input from each eye.
• CO blobs are the regular, blob-shaped groups contained in striate cortex (spread across 0.5 millimeters); they are thought to be areas for gathering/combining information.

What Happens When You Get Too Much Stimulation?

• Adaptation is a phenomenon where visual response to constant stimulation decreases over time.
• In this state, cells become less likely to fire and they are temporarily less active.

End Stopping

• Receptive field responses are strongest to bars of a certain length.
• The response weakens when stripe length changes beyond a certain range, shorter or longer compared to that ideal length.

Columns

• Columns are vertical structures of neurons; all cells have the same preferred orientation and similar receptive fields.
• Orientation changes along the cortical columns approximately 10 degrees every 0.05 mm (horizontal).
• All orientations are covered within a space of roughly 0.5 mm.
• Columns can indicate orientation as well as the ocular dominance factor.

Hypercolumns and Blobs (Cont)

• Each column has a specific orientation preference.
• Adjacent groups of columns display a specific ocular dominance.
• Blobs are cubic shaped columns visible in striate cortex.

Simple and Complex Cells (V1 physiology)

• Simple cells have clearly defined excitatory and inhibitory fields; firing is triggered by specific oriented bars.
• Complex cells do not have well-defined fields, and may be more sensitive to the positioning of the stripe, the spatial frequency and/or the ocular dominance

Simple and Complex Cells (Cont)

• Simple cells only fire efficiently if a bar is oriented within the center of their receptive field
• Complex cells continue generating strong signals even if the stripe is not placed within their receptive field, but the precise orientation, spatial frequency, and the eye dominance factors are essential for complex cells to fire well.

Receptive Field Tuning

• Cortical cells respond to the gratings and display a narrower spatial frequency tuning compared to retinal ganglion cells.
• These can be tuned to oriented bars, with different responses to moving lines, edges, or specialized bars.

Ocular Dominance in V1

• Separate processing for the left and right eye information for each cell in the V1 region.
• Each cell may have preference to one eye more than the other, but not exclusively.

Adaptations to Spatial Frequency

• Selective adaptation shows an observed change to spatial frequency characteristics.
• It suggests evidence that cells in the striate cortex are specially/tuned to respond best to specific spatial frequencies.

Spatial Frequency Analyzers

• Our visual perception and processing depends on and incorporates multiple cells specialized to different spatial frequencies.
• This integration of responses allows the visual system to perceive a complete and holistic image.

Different Spatial Frequencies Provide Different Information

• The spatial frequencies, in visual perception, provide different data about a visual scene.
• High frequencies give fine details, whereas low frequencies contribute to broad outlines and/or context.

High-Frequency Masks

• High spatial frequencies can mask low spatial ones, helping the brain focus on the most critical elements in a visual scene—the high spatial frequencies.
• Without low frequency (context) information, the outlines and features are not easy to perceive fully.
• Spatial frequency filters or masks are easily overcome with squinting to improve the perception of a visual field

Tune in Next Time...

• The next session will cover perception and recognition of objects, including potential errors or issues during the process.