Perception of Stimuli Stephen Taylor

Processing Visual Stimuli The lens focuses light onto the retina at the back of the eye, where it stimulates photoreceptors (rods, sensitive in low light with low acuity; and cones, sensitive to colour in high light, with high acuity). Photoreceptors synapse with bipolar neurons. These feed into ganglion cells, carrying the impulse to the visual cortex through the optic nerve. Some ganglia are sensitive to impulses from the edge of the receptive field, where others are sensitive to impulses from the centre. Edge enhancement (due to lateral inhibition of cells in the retina) results in greater contrast around edges. Stimulus from the left visual field of each eye is processed in the right side of the brain and vice versa. This is due to contralateral processing via the optic chiasm Uses the retina and the brain. Thanks to John Burrell & David Mindorff

Rod CellsCone Cells Many rod cells feed into one ganglion: all their action potentials are combined into a single impulse at the synapse. This means each ganglion has a large receptive field, but low acuity (low ability to detect differences). Rod cells are activated in low light conditions, but ‘bleached’ in high light intensities. They do not detect colour. Rods are distributed throughout the retina. Cone cells feed into their own ganglion. This gives a small receptive field for each ganglion, leading to high visual acuity – small differences are easily detected. There are three types of cone cells, receptive to different wavelengths (red, green, blue). These are only active in sufficient light. Cone cells are concentrated in the fovea. images adapted from

images adapted from Receptive Fields and Processing Visual Stimuli Many rod cells feed into one retinal ganglion. This means that many impulse converge to form a single signal which is sent to the brain. There is no distinction between stimuli which hit different sections of the same receptive field. Some ganglia are stimulated by impulses sent from rod cells from the edge of their receptive field and inhibited by signals from the middle. Other ganglia are inhibited by impulses sent from rod cells from the edge of their receptive field and stimulated by signals from the middle. This allows for greater perception of contrast. Edge enhancement also plays a key role.

Explaining Edge Enhancement Although each band is uniformly shaded, regions around the edges are enhanced in your vision. appears darker appears lighter retina Light hits the photoreceptors. More light, more stimulation. In these diagrams, as the receptor cells get brighter, is shows a stronger signal. Stimulated photoreceptors pass the action potential to the bipolar neuron and ganglion. uniform signal

Explaining Edge Enhancement Although each band is uniformly shaded, regions around the edges are enhanced in your vision. appears darker appears lighter retina Light hits the photoreceptors. More light, more stimulation. Neighbouring cells will inhibit the neurons of each other. Greater stimulation of the receptor means greater inhibition of the neighbours. This is called lateral inhibition. If all neighbouring cells receive the same stimulus (and therefore inhibition), they will produce a uniform signal. Stimulated photoreceptors pass the action potential to the bipolar neuron and ganglion. uniform signal

Explaining Edge Enhancement Although each band is uniformly shaded, regions around the edges are enhanced in your vision. uniform weak signal (dark colour perceived) uniform strong signal (light colour perceived) stronger signal: brighterweaker signal: darker If an edge falls within a visual field, edge enhancement occurs. Receptors receiving a stronger stimulus will inhibit their neighbours more strongly, and vice- versa. So a neuron that is more inhibited than its neighbours will result in a darker colour being perceived (on the dark side of the edge), and vice versa, giving an enhanced contrast on the border between light and dark images.

Explaining Edge Enhancement ABCD Why is B darker than A? A receives the same weak stimulus as its neighbours and so is inhibited equally by them. B is next to C, which recieves a stronger stimulus and therefore inhibits C more. As a result, B is overall more inhibited than A, so is darker. Why is C brighter than D? D receives the same strong stimulus as its neighbours and so is inhibited equally by them. C is next to B, which recieves a weaker stimulus and therefore inhibits C less. As a result, C is overall less inhibited than D, so is brighter. Receptor A receives the same light stimulus as B. Receptor D receives the same light stimulus as C.

It’s more like a gradient… see if you can explain why by annotating the diagram.

images adapted from

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