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Receptive Fields
React differently to different inputs


▪ The receptive field of a neuron is the area of the visual field in which the presence of a stimulus influences the firing rate of that neuron,
i.e. the part of space in which the light must fall for the neuron to be stimulated.
▪ Microelectrodes are used to record electrical activities of single neurons.
▪ To find a receptive field, the investigator can shine light in various locations while recording from a neuron.
▪ If the light from a particular spot excites the neuron then that location is part of the neurons excitatory receptive field (ON firing).
▪ If the light inhibits activity, the location is in an inhibitory receptive field (OFF firing).

1

ON and OFF Firing


▪ Neurons respond with either ON firing or OFF firing depending on the location
of the spot of light in the receptive field.
Stimulation of the central field (centre) or the surrounding field (surround) have contrary effects.
ON cells are excited by light falling in the centre but inhibited by light falling in the surround.
OFF cells are excited by light falling in the surround but inhibited by light falling in the centre.

It is a schematic, not a perfect round shape

2

Seeing Edges (The Perception of Contrast)


▪ A visual edge is a place where two different areas of a visual image meet.
▪ The centre-surround organisation of receptive fields enhances our ability to detect the outlines of objects when the contrast between the object and background is low (contrast enhancement).
▪ When a receptor fires, it inhibits its neighbours via a lateral neural network. This is known as lateral inhibition because it spreads laterally across an array of receptors.

3

Lateral Geniculate Nucleus (LGN)

▪ The retina-geniculate- striate-pathway conducts signals from the retina to the primary visual cortex (striate cortex or area V1) via the lateral geniculate nucleus (LGN).
Lateral Geniculate Nucleus (LGN)
The LGN comprises six layers of neurons:
▪ Layers 1 and 2 have large cell bodies and large receptive fields (inner
magnocellular layers).
▪ Layers 3-6 have small cell bodies and small receptive fields (outer
parvocellular layers).

4

The Retina-Geniculate-Striate Pathway (The Primary Visual Pathway)


▪ The optic nerves join at the base of the brain to form an x-shaped optic chiasm.
▪ The axons from the ganglion cells serving the inner half of the retina (nasal sides), cross through the chiasm and ascend to the LGN of the opposite side of the brain.
▪ The axons of the ganglion cells serving the outer half of the retina (temporal sides) remain on the same side of the brain.
▪ Each hemisphere receives information from the contralateral visual scene/eye.

5

Orientation-Sensitive Neurons
(Hubel and Wiesel, 1960’s)


An orientation-sensitive neuron in the striate cortex will respond only when a line of a particular orientation appears within its receptive field.
This cell fires most when a vertical line is presented in the receptive field

6

Response Characteristics of a Orientation Neurons
Simple cells


Neurons whose receptive field is organised in an opponent fashion
Simple cells
Neurons whose receptive field is organised in an opponent fashion
Complex cells
Neurons which do not have an inhibitory surround; they respond when line moves perpendicular to its angle of orientation
Hypercomplex cells
Neurons that have inhibitory regions at the end (or ends) of a line segment, used to detect obscure objects

7

Spatial Frequency and Sine-Wave Gratings
(DeValois et al., 1978)

Sine-wave grating
▪ A sine-wave grating is a set of equally spaced, parallel, alternating light and dark stripes that vary in brightness.
▪ The sine-wave is designated by its spatial frequency which is the relative width of the stripes/bands measured in cycles per degree of visual angle.
▪ The visual angle is smaller if the sine- waves are closer together

Shows cells have multiple sensitivity depending where stimulus is in field. Giving info about small objects in env. High frequency.
Large area means low frequency.

Sharp edges provide a signal in high frequency
Images deficient in high frequency information look unfocused but we can still make out the form
Lincoln in ex.

8

Retinal Disparity

Fuzziness can be detected in monocularly, but sterscopis requires both.
Neurons respond to visual stimulation in one eye.
Neurons respond to visual stimulation in each eye.
The perception of depth that emerges from the fusion of two slightly different projections of the image on the two retinas. The difference between the two eyes' images, which is a result of the eyes' horizontal separation, is usually referred to as binocular disparity or retinal disparity.

9

Visual Association Cortex


▪ The extrastriate cortex is a layer of neurons that surround the striate cortex. (beyonde area v1) It consists of several regions that form independent maps.
▪ Each region responds to specific features of the visual environment.
▪ Outputs of striate cortex (area V1) are sent to areas of extrastriate cortex (area V2/V3/MT) in a hierarchical fashion.

10

What’ and ‘Where’ Visual Streams
(Ungerleider and Mishkin, 1982)


The ventral (what) stream recognises what the object is and its colour
The dorsal (where) stream recognises where the object is and whether it is moving.

11

Perception of Form


▪ The inferior temporal (IT) cortex (the end point of the ventral visual stream) consists of area TE area TEO. Takes whole contra lateral area.
▪ The analysis of visual information is hierarchical; the receptive fields of neurons increase in successive regions as the visual image becomes more complex.
▪ The receptive field of neurons in area TE are larger than those of area TEO.
▪ These cells respond well to 3D-objects and continue to respond when the image moves to different locations, changes in size, is partially occluded by other objects, etc.
▪ Thus, cells in IT participate in recognition of objects rather than analysis of specific features. Involves learning.

12

Effects of damage to the dorsal and ventral stream


Visual agnosia is the inability to recognize objects, persons or shapes
in the absence of blindness or memory loss.
Patient D.F. has injury to the ventral stream
Patient R.V. had injury to the dorsal stream – optic ataxia
Focal. Where and motor stream issue.

13

Prosopagnosia, face blindness.


Loss of face identification. Feature integration.

Special face recognising circuits are found in the fusiform gyrus (in the inferior temporal cortex)

Implied faces activate the fusiform gyrus

14

Motion Perception
Importance, area _____?


Area MT (or area V5) of the extrastriate cortex contains neurons that
respond to motion.
Area MT receive input directly from the striate cortex as well as the
superior colliculus (in tectum) which is involved in visual reflexes.
Area MST (or area V5a) performs further motion analysis such as
radial, circular and spiral motions.
Optic flow is the analysis of the relative movement of the visual elements around us. It provides information about objects in the environment as we move around or as objects move around in relation to us.
Bilateral damage to area are MT leads to akinetopsia; the inability to perceive movement.

Form from Motion (Johansson, 1973)

Perception of Motion and Perception of Form: Neuropsychological evidence
Although motion is processed by MT/MST, the ability to detect form from motion is processed in the right medial occipital lobe.
- Patient R.A. (Vaina , 1998) could perceive motion (optic flow) but could not perceive form from motion.
- Patient L. M. (Zihl et al., 1991) could recognise form depicted solely by the moving points but she could not perceive motion.