Visual perception II: Higher-level vision Flashcards
Simple feed forward model of simple cell selectivity
A bar of light at the orientation of an ON subfield that is moved or flashed within the subfield will simultaneously activate all of the presynaptic geniculate ON-center cells. The resulting barrage of synaptic excitation will depolarize the cortical cell and cause it to fire spikes. In contrast, a bar moved or flashed at right angles to the subfield will only activate a small subset of the underlying geniculate relay cells at one time. The resulting depolarization of the simple cell would be too small to reach threshold, leaving the simple cell inactive.
Gabor Filter
a model of simple cell responses in V1. It constitutes a sinusoid multiplied with a Gaussian window.
End Stop Cells
End stop cells are only responsive to a line of a certain length or to a corner of a larger stimulus. These cells have a reduced or absent response when the line or corner is extended beyond a certain point. End stopping is a characteristic of hypercomplex cells in striate cortex.
What are “Line Detectors”?
Unlike many of the cells in the retina, which respond to spots of light or dark, Hubel and Wiesel found that cells in the visual cortex were highly selective for edges (or lines) of a specific orientation
Direction Tuned Cells
Cells distinguished between direction but orientation remains the same.
Simple Cells
Simple cells are defined by the elongated ON and OFF subfields into which their receptive fields can be divided. These subfields are arranged side-by-side, with their long axes parallel to the axis of the preferred orientation of the cell.
Why is a Gabor filter considered to be a model of V1?
Cells in V1 respond to the spatial frequency and orientation of Gabor filter
Cortical Columns
Cortical columns are structures that group together cells that have a similar tuning pattern.
What is a complex cell?
Complex cells are also neurons in V1 that respond optimally to a stimulus with a particular orientation. But, unlike simple cells, they respond to a variety of stimuli across different locations. For example, a complex cell will respond to a dark bar on a light background and a light bar on a dark background. In contrast, a simple cell only responds to one but not the other. Moreover, complex cells do not have peak location sensitivity as simple cells do. That is, they will respond equally well to an optimal orientation regardless of where it is within the cell’s receptive field. Complex cells also respond best to moving stimuli.
Phase Invariant
Does not matter where bars are presented inside receptive field. The receptive field cannot be mapped by linear models
Simplifying Assumption: Linearity
Formula to simplify what a neuron in the visual field responds to:
R= W1 i1 + W2 i2 + W3 i3 + …. Wn in
Wn = filter weight at location n in= intensity at location n
*Works with simple cells
Violation of Linearity: Complex Receptive Fields
Location only triggers a response if the stimulus in all the other locations is in the correct location
Contextual Modulation
Refers to the influence of a surround pattern on either the perception of, or the neural responses to, a target pattern.
What is the early “Ice Cube Model”?
The ice cube model describes the structure of V1 cortical tissue. It assumes that V1 can be separated into orderly arranged cubes in which all information from one part of the visual field is processed.
First, the cube is arranged in orientation columns containing neurons with similar orientation selectivity, so that in the vertical extent, there is no change of response properties, whereas along one tangential axis, there is a continuous change for preferred orientation. Second, in addition to columns of cells sensitive to different orientations, cells in V1 are also organized into alternating ocular dominance columns, which contain cells that are responsive only to input from the left or right eye. Third, cells responding to color are organized into regions called blobs
What does the center of the pinwheel signify?
Orientation insensitive column of cells.
What is a “Pin Wheel Structure” in orientation tuning?
A pin wheel structure is the location where multiple orientation columns converge. Orientation columns are organized radially around a point known as a singularity. The arrangement, around the singularity, can be observed to be in both a counter-clockwise or clockwise fashion.
What is cerebral akinetopsia?
The inability to perceive fast movement, despite spared perceptions of static images
Part Retinotopic Mapping…
Stimulating upper part of visual cortex; light sensation in lower visual field
Stimulating upper part of visual cortex; light sensation in the lower visual field
BREAK TIME :)
Make a coffee ☕️
