Visual Perception - 2

Regions of the visual field in which light stimulation causes the receptor to respond

Yes, despite not resopnding to light (response driven by photoreceptors)

Light around a cell's RF has opposite effect to light in cell's RF centre
Cells respond to relative stimulation of centre vs periphery rather than overall light stimulation
Retina responds poorly to large uniform regions of light

Hartline and Graham 1930s in a horseshoe crab
Kuffler 1950s in the vertabrete eye of a cat

Excitatory inputs at centre of cell and inhibitory inputs in surround

Inhibitory inputs at centre of cell and excitatory inputs in surround

Simultaneous brightness contrast
Mach bands in stepped-intensity patterns

Simultaneous brightness contrast

Blue represents surround of retinal ganglion cell
Red represents centre of retinal ganglion cell
Our perception is the relative contrast between the centre and surround of RF
Eg the right ganglion cell
Dark centre with lighter surround
Firing will be inhibitied so rectangle will look darker

Stepped-intensity patterns

All centre cells = medium grey
Left ganglion cell: more light hitting surround > centre = inhibition of responses
Right ganglion cell: surround darker > centre = cell activated
Changes in brightness - makes it look like there are edges

Mostly contrast at edges

Orientation
Colour

Neurons are 'tuned to' or 'selective for' specific visual features
Principle of univariance
Principle of adaptive independence

Particular wavelengths of light
Particular orientation of a bar

Bars of different orientations presented to a cell's RF
Cell tuned to 45degrees in this example

Cells responses vary along a single dimension - firing rate can increase/decrease
BUT multiple factors can influence firing rate (eg. orientation, salience)
So cannot use firing rate in isolation to understand what is in the cell's RF
Response of a single neuron is ambiguous (seen in image)

Pattern coding
Compare output of several cells

Relative responses of mechanisms tuned to different features provide an unambiguous signal (eg. relative response ratio of S/M/L wavelength cones in colour vision)
This arises for orientation, depth, motion etc.
Combinging responses of multiple cells

Neurons selective for 1 stimulus can be selectively adapted without other neurons being adapted
Leads to perceptual distortions
Can use this to make inferences about underlying neurophysiology

Individual mechanisms can be adapted independently
Neurons selective for 1 stimulus can be selectively adapted without other neurons being adapted
Useful for revealing visual mechanisms
Can independently adapt cells tuned to certain features (without affecting other cells)

Afterimages - perception of clear images on blank background after stimulation of eye has ceased
Aftereffects - adaptation to stimulus alters our perception of a second stimulus
Adapting to stimulus often leads ot negative afterimages/aftereffects

Respond equally to an edge/line of any orientation

Outputs of several ganglion cells combiend with neighbouring RFs
Responses summated

Use perceptual effects of adaptation
Adapt pp to lines orientated 10-15degrees from vertical
Causes misperceptiono f second patch of vertical lines (appear to be tilted in opposite direction to adapted stimulus) - negative tilt aftereffect
Sharp peak in aftereffects arise at 10-15degres separation between adapting and testing angles
Suggest orientation units in visual system tuned to orientation 10-15 degrees apart might mutually inhibit each other
Tilt aftereffects may reflect inhibitory interactions between neighbouring regions in V1 (helps to finetune perception of line's orientation)

Reveals that orientation is pattern coded

1959
Made recordings from primary visual cortex in macaques
Found within functional units of cortex, known as 'hypercolumns' cells in neighbouring regions tended to code orientations 10-15degrees apart
Neighbouring orientation columns in V1 'tuned to' orientations 10-15 degrees apart

Relative firing of several orientation-sensitive neurons coding a particular area of the visual field
Type of pattern coding