Perceptual Processes ch 3

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Questions and Answers

Where does information from the photoreceptors travel to after passing through the retinal ganglion cells?

Striate visual cortex

Lateral geniculate nucleus of the thalamus (correct)

Primary visual cortex

Occipital lobe

What does the 'WHAT' pathway refer to?

The input from M cells

The dorsal stream involved in processing spatial information

The ventral stream involved in processing object recognition (correct)

The input from P cells

What are cells in the visual cortex preferentially responding to ?

Points of light

Lines, edges, bars, and stripes (correct)

Luminance differences

Grating cycles

What is the smallest spatial detail resolved at 100% contrast?

Acuity (A) Signup and view all the answers

What is the visual angle?

The angle subtended by an object at the retina (B) Signup and view all the answers

How is visual acuity characterized by eye doctors?

Distance at which a person can see a particular object (E) Signup and view all the answers

Who invented the method for designating visual acuity?

Herman Snellen (B) Signup and view all the answers

What is the relationship between visual acuity and the visual angle of a grating cycle?

The smaller the visual angle, the greater the visual acuity (A) Signup and view all the answers

What is the approximate distance between columns with different orientation tuning in the striate cortex?

0.5 mm (C) Signup and view all the answers

What is the approximate change in orientation angle every 0.05 mm moved horizontally in the striate cortex?

10 degrees (D) Signup and view all the answers

What is a hypercolumn in the striate cortex?

A group of columns representing all possible orientations and ocular dominance (A) Signup and view all the answers

Which of the following is NOT a characteristic of CO blobs?

They are primarily involved in processing orientation information (D) Signup and view all the answers

What is the primary function of adaptation in the visual system?

To reduce neural activity in response to prolonged or repeated stimulation (A) Signup and view all the answers

How can adaptation be used to selectively "knock out" groups of neurons?

By presenting a stimulus for an extended period of time, causing the neurons to adapt and stop responding (C) Signup and view all the answers

What perceptual illusion demonstrates the existence of neurons selective for different orientations?

The tilt aftereffect (A) Signup and view all the answers

Why does the visual system analyze images using spatial frequency filters?

To improve the recognition of objects by breaking them down into their component parts (C) Signup and view all the answers

What is the primary function of the LGN?

To process visual information from the retina and send it to the visual cortex (A) Signup and view all the answers

How does the LGN differ from the retina?

The LGN receives information from both eyes, while the retina receives information from only one eye. (B) Signup and view all the answers

What is the main function of the magnocellular layers of the LGN?

Processing of movement and depth (B) Signup and view all the answers

Which layer of the LGN receives input from the contralateral eye?

Layers 1, 3, and 6 (D) Signup and view all the answers

What is the primary difference between circular receptive fields found in the retina and LGN compared to the elongated 'stripe' receptive fields found in the striate cortex?

Elongated receptive fields are more sensitive to movement and orientation. (D) Signup and view all the answers

Where do the fibers from the LGN primarily project to in the striate cortex?

Layer 4C (C) Signup and view all the answers

What is meant by 'magnification' in the context of cortical mapping?

Different regions of the visual field are represented by different sizes of areas in the cortex. (B) Signup and view all the answers

How does the striate cortex differ from the LGN in terms of cell number?

The striate cortex has significantly more cells than the LGN. (C) Signup and view all the answers

What is the relationship between receptive field size and the perception of stripes?

Smaller receptive fields are needed to perceive stripes, while larger receptive fields perceive a uniform grey. (C) Signup and view all the answers

What is the connection between the center-surround cells and the perception of boundaries?

Center-surround cells are arranged in columns and rows, allowing for recognition of boundaries. (A) Signup and view all the answers

How does resolutional acuity relate to the perception of detail?

Resolutional acuity is the finest detail that can be resolved, determined by the density of photoreceptors. (B) Signup and view all the answers

What is the relationship between spatial frequency and contrast sensitivity function (CSF)?

Spatial frequency influences the shape of the CSF. (A) Signup and view all the answers

What is the significance of the sine-wave grating in studying visual perception?

Sine-wave grating allows for controlled manipulation of spatial frequency. (A) Signup and view all the answers

How does myopia affect contrast sensitivity?

Myopia reduces contrast sensitivity, especially in severe cases. (B) Signup and view all the answers

What is the relationship between the fovea and the periphery in terms of receptive field size?

Fovea has smaller receptive fields than the periphery. (C) Signup and view all the answers

What is the role of horizontal and vertical distribution of photoreceptors on receptive field size?

Vertical distribution has a greater impact on receptive field size, especially below the midline. (D) Signup and view all the answers

How does the adaptation level of the eye affect the contrast sensitivity function (CSF)?

The adaptation level can shift the entire CSF curve. (C) Signup and view all the answers

What is the implication of the fact that individual contrast sensitivity functions (CSFs) vary?

Individual CSF variations suggest that our ability to perceive contrast can differ significantly. (B) Signup and view all the answers

What type of stimulus did Hubel and Wiesel find caused the most firing in the cat's visual cortex?

Stripes or edges (C) Signup and view all the answers

What is the name of the area of the brain where neurons respond to oriented bars of light?

Striate cortex (C) Signup and view all the answers

What is the difference between simple and complex cells in the striate cortex?

Simple cells respond to bars of light in a specific location, while complex cells respond to bars of light in any location within their receptive field. (C) Signup and view all the answers

How does the brain create neurons that respond to specific orientations of light?

By combining the inputs from multiple neurons in the LGN that respond to different orientations. (A) Signup and view all the answers

Which of the following is NOT true about receptive fields in the visual system?

Receptive fields in the LGN are responsible for orientation selectivity. (D) Signup and view all the answers

Which of the following is NOT a factor that a complex cell is sensitive to?

Position (A) Signup and view all the answers

In terms of receptive fields, what is the difference between simple and complex cells?

Simple cells have clearly defined excitatory and inhibitory regions, while complex cells do not. (D) Signup and view all the answers

What does it mean for a cortical cell to have "ocular dominance"?

The cell responds more strongly to input from one eye than the other. (C) Signup and view all the answers

What is the function of 'end stopping' in neurons?

To increase the neuron's sensitivity to specific lengths of gratings. (D) Signup and view all the answers

What is a 'column' in the context of the visual cortex?

A vertical arrangement of neurons with similar receptive field properties. (B) Signup and view all the answers

Which of the following statements is TRUE about the receptive fields of simple and complex cells in the visual cortex?

Simple cells are sensitive to the position of a stimulus in the receptive field, while complex cells are not. (C) Signup and view all the answers

Why do cortical cells have narrower spatial frequency tuning than retinal ganglion cells?

Because cortical cells are further from the retina and have received more input from other neurons, leading to a more specific response. (A) Signup and view all the answers

What is the primary function of LGN cells as described in the text?

To respond to input from only one eye, never both. (C) Signup and view all the answers

Study Notes

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.

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Test your knowledge on visual perception and processing concepts, including the roles of photoreceptors, retinal ganglion cells, and the visual cortex. This quiz covers critical aspects such as visual acuity, orientation tuning, and adaptation in the visual system. Perfect for students studying neuroscience or psychology.