Cognitive Neuroscience - Vision

The retina has 3 layers of cells: the [blank_start]photoreceptor layer[blank_end], [blank_start]bipolar cell layer[blank_end] and [blank_start]ganglion cell layer[blank_end].

While [blank_start]rod[blank_end] cells or the retina are responsible for dim light vision, [blank_start]cone[blank_end] cells of the retina are responsible for daytime vision.

The optic nerve crosses at the [blank_start]optic chiasm[blank_end], such that each side of the brain receives visual information from the opposite side of the [blank_start]visual field[blank_end].

The [blank_start]lateral geniculate nucleus[blank_end] of the [blank_start]thalamus[blank_end] is the primary relay nucleus for visual processing.

In the context of visual processing, the receptive field of a neuron refers to the region in the [blank_start]visual field[blank_end] that elicits a response from that particular neuron.

When a spot of light falls on the centre of the receptive field of an ON (ganglion) cell, the cell will be [blank_start]excited[blank_end]; when a spot of light falls on the surround of its receptive field, the cell will be [blank_start]inhibited[blank_end].

When a spot of light falls on the centre of the receptive field of an OFF (ganglion) cell, the cell will be [blank_start]inhibited[blank_end]; when a spot of light falls on the surround of its receptive field, the cell will be [blank_start]excited[blank_end].

When light of the same level of luminance covers the entire receptive field of an ON or OFF cell, the firing rate of the cell will [blank_start]not change[blank_end].

the contrasting centre-surround configuration of retinal ganglion cells enables the detection of [blank_start]edges or abrupt changes in brightness[blank_end].

[blank_start]Short-wavelength/blue[blank_end] cones respond maximally to short wavelength light or blue light. [blank_start]Medium-wavelength/green[blank_end] cones respond maximally to medium wavelength light or green light. [blank_start]Long-wavelength/red[blank_end] cones respond maximally to long wavelength light or red light.

The ratio of the activity of three types of cone cells enables the coding of [blank_start]colours[blank_end] at the [blank_start]photoreceptor[blank_end] layer of the retina.

Neurons in the [blank_start]primary visual cortex[blank_end]/[blank_start]striate cortex[blank_end]/[blank_start]area V1[blank_end] of the cerebral cortex detect simple features of the visual scene, such as line orientation, colour and movement.

In area V1, [blank_start]simple[blank_end] cells are bar detectors; [blank_start]complex[blank_end] cells are movement detectors; and [blank_start]hypercomplex[blank_end] cells are edge detectors.

Area V4 specialises in [blank_start]colour vision[blank_end].

Area V4 enables [blank_start]colour constancy[blank_end], which is referred to as the ability to recognise the colour of an object despite changes in lighting.

Area V5/MT contributes to [blank_start]movement perception[blank_end].

Visual information leaves the occipital lobe via two streams or pathways: the [blank_start]dorsal[blank_end] stream consists of projections to posterior parietal regions, and the [blank_start]ventral[blank_end] stream consists of projections to inferior temporal regions.

The [blank_start]ventral[blank_end] stream of the visual system is specialised in object recognition, whereas the [blank_start]dorsal[blank_end] stream is specialised in locating objects in space and guiding movements.

When outlining the visual processing in the brain, the [blank_start]striate cortex[blank_end] is where the visual scene is analysed in small segments, as each module within the area only focuses on one small portion of the [blank_start]visual field[blank_end]. In the striate cortex neurons detect simple features such as [blank_start]line orientation and movement[blank_end]. The [blank_start]extrastriate[blank_end] regions provide the role of divide and conquer whereby these regions specialise in certain functions, such as [blank_start]V4[blank_end] for colour vision and [blank_start]V5[blank_end] for movement perception. The visions are then projected from the [blank_start]extrastriate[blank_end] regions to other cortical areas via two separate streams: the [blank_start]ventral[blank_end] stream and the [blank_start]dorsal[blank_end] stream. The ventral stream ('[blank_start]what[blank_end]' pathway) is where simple features are combined into wholes for [blank_start]object recognition[blank_end]. The dorsal stream ('[blank_start]where[blank_end]' and '[blank_start]how[blank_end]' pathway): serves to [blank_start]locate objects in space[blank_end] and guides actions.