Questions for Exam 1
1) What is the importance of convergence in retinal processing? What tradeoffs does
this impose for visual functions?
Convergence: 5 million cones, 120 million rods, 2 million ganglion cells
Therefore, many receptors must send signals to each ganglion cell and this is
The extent of convergence on different parts of the retina produces a tradeoff
between sensitivity to low light levels and sensitivity to fine spatial detail.
There is greater convergence of rods (sensitive to light) than cones (finer details)
onto ganglion cells. Thus, there is a greater summation of rod signals and less
stimulation per rod is required to obtain a response.
Greater convergence of rods than cones onto ganglion cells limits their spatial
2) What is lateral inhibition? Describe two perceptual phenomena that can be
explained by this concept.
The response properties of retinal ganglion cells can be explained by the neural
circuit depicted above. Note that the neuron being recorded receives excitatory
inputs from one group of receptors and inhibitory inputs from receptors in
For an on centered ganglion cell, the response rate is greatest when the stimulus
just fills the excitatory central region. When the stimulus covers the entire
receptive field the cell will fire at its background rate.
Ganglion cells that receive inputs from the fovea have smaller receptive fields
than cells that receive inputs from more peripheral regions.
Simultaneous Lightness Contrast: Gray Square looks darker on light background.
-The right square appears darker because cells with receptive fields near
its borders receive less inhibition from the surround.
Herman Grid: Spots appear at the junctions, but disappear when fixated on. 3) Describe the different types of retinal ganglion cells on the retina. What are the
properties by which they can be distinguished?
-P Cell’s Sustained Response: when a stimulus is turned on, P cells respond
continuously until the stimulus is turned off. P= Parvo
-M Cell’s Transient Response: When a stimulus is turned on, M cells respond for
a short time only and then return to their background level. Response rate is
decreased when the stimulus is turned off. M= Magno
-K Cells lay in between the rows of P or M cells.
4) Describe the differences between simple, complex and end stopped cells in the
-Simple cells: The receptive fields of simple cells have excitatory and inhibitory
areas arranged side by side. This type of cell responds best to lines or bars at a
particular orientation. Simple cell receptive fields can be produced by combining
the outputs of LGN cells. Simple cells are also tuned to the width of a bar, and the
optimal width varies with the size of a cell’s receptive field.
-Complex cells: Respond best to lines or bars at a particular orientation that are
moving in a specific direction. Complex cell receptive fields can be produced by
combining the outputs of simple cells.
-End-Stopped cells: Respond best to lines or bars at a particular orientation that
do not extend outside the boundaries of its excitatory receptive field. End-stopped
cell receptive fields can be produced by combining the outputs of simple cells.
5) What are the dorsal and ventral streams? Describe three sources of evidence that
have helped to reveal the functions of these pathways.
Beyond V1, these are separate retintopic maps in areas V2 and V3, and the cells
in these areas are similar to those in V1.Areas V1, V2, and V3 are referred
to collectively as the primary visual cortex.
From the primary visual cortex, information is processed along two functionally
distinct pathways: one that flows dorsally to the parietal cortex and another that
flows ventrally to the temporal cortex.
-Patients with lesions of the dorsal stream have difficulty in pointing or
FMRI studies he talked about
Evidence from making lesions in monkeys Studies from brain damage
Single cell electrophysiology do work on single cells in brain
-Balint’s syndrome (dorsal lesion): patient has poor control of
visually guided movement and is unable to identify two
objects at once.
-Patients with lesions of the ventral stream have difficulty in recognizing
objects or faces.
-Appreceptive agnosia (ventral lesion): Patient is unable to
recognize or draw objects from visual presentations
-Associative agnosia (ventral lesion): Patients can perceive intact
objects and draw them, but is unable to name objects from
-Prosopagnosia (ventral lesion): Patient is unable to identify faces,
though they can still recognize other types of objects.
Visual Streams – What/ Where: Ungerleider & Mishkin (1982)
A. Monkeys with lesions in the temporal cortex are impaired at
discriminating objects (they fail to consistently pick out the
pyramidal object for the reward). The “What” pathway is the ventral
B. With Posterior Parietal Lesions they fail to perform on location tasks
(pick the object near the post). The “Where” pathway is the dorsal
Visual Streams- What/ How: Milner and Goodale 1991
-Worked with a patient, D.F. with extensive bilateral ventral-stream
lesions since 1991
-D.F. has profound visual form agnosia
-But some interesting spared abilities
-Balint’s syndrome- dorsal lesions: optic ataxia (poor control of visually-
6) What are the differences between motion processing in the primary visual cortex,
the medial temporal cortex, and the medial superior temporal cortex?
7) What is Fourier analysis? What is the evidence that a similar process occurs in
Fourier analysis is the analysis of a complex waveform expressed as a series of
sinusoidal functions, the frequencies of which form a harmonic series.
This is also true for vsual patterns.Any visual image can be created by adding
together sine waves with the appropriate contrasts, orientations and spatial
frequencies. Acommon stimulus is gratings (alternating light and dark bars).
Add together in the right way you can reproduce any noise
Single cells in cortex they respond to specific frequencies; selective adaptaion studies on
adapt humans, measure sensitivity pre/post adaptaion, subtract and get a curve simial to
Complex signal, reproduce by adding together simplier signals.
8) What is the contrast sensitivity function? How can the effects of selective
adaptation on this function tell us about the properties of simple cells in the visual
cortex? The contrast sensitivity function (CSF) is a plot of the threshold contrast to detect
the grating (as opposed to seeing a uniform gray) as a function of spatial
-For photopic (daylight) vision, the CSF peaks around 2 to 4 cycles per
degree. Note how sensitivity is reduced for mesopic (twilight) or
scotopic (nighttime) vision.
-Contrast sensitivity is quite poor at birth, but improves gradually with
-Contrast sensitivity is reduced with aging, primarily for high spatial