Lecture 6 Visually Guided Actions

imissyou419's version from 2017-10-28 03:13


Question Answer
Is the ventral stream subject to perspective illusions?Yes
Is the dorsal stream subject to perspective illusions?No. You want to direct actions to precise locations.
What frame of reference does ventral streams use?object-centered frame
What frame of reference does the dorsal stream use?egocentric reference
1. early visual areas code objects with respect to the retina,
2. early auditory areas code locations with respect to the head,
3. location of finger is coded with respect to body
Lesion in dorsal streamdifficulty in making sacchades, grasps, reaches, and feeding accurately
Intra Parietal Sulcus (IPS)ideally located to integrate the representation of space that are derived from several modalities of sensory info: visual, somatosensory, and auditory. Locations can be seen, felt, heard. IPS contain several fields each responsible for directing a particular type of movement
Parietal Eye Field (PEF)represents the retinotopic locations of objects that you intend to look at/give attention to. Here neurons respond to visual and auditory stimuli that indicates a location
Parietal Arm Field (PAF)represents the retinotopic locations in the immediate peripersonal space (the region of space one can reach to). PAF is used to direct arm movements. Neurons receive both visual and somatosensory info. PAF is elastic
Lesion in PAFoptic ataxia (visually directed reaching errors even though vision and arm motor systems are functioning)
Parietal Grasp Field (PGF)represents not the location of objects, but the shape information required to grasp objects. Also important is object information. Receives input from the ventral space of an object's properties such as their allocentric co-ordinates to grasp a knife by the handle and not the blade.
Parietal Face Field (PFF)represents the ultra space that is used to guide the head, mouth, and lips during feeding or kissing. Neurons receive visual and tactile/somatosensory input from the face. Neurons in PFF code specific locations with respect to the head.
The same neuron is activated by touch to a particular body part, sight of it approaching that body part from any direction independent of gaze direction but NOT activated by sight of an object approaching another body part (job of other PFF neuron)
Short latency saccadesSaccades to a novel peripheral stimulus (flashing/moving) involve the SC; eye -> SC -> eye motor command
Long latency voluntary saccadesPrefrontal association area holds the location of remembered targets in working memory and makes decision that one is of interest. The PEF directs attention at the one of interest. And the frontal eye field (FEF) generate a long latency saccade to it
Hills of activity in the SCmagnocellular ganglion cells have large receptive fields and project to SC so activity is not localized to a point but to an area where the center has highest activity.
Visual grasp reflexvisual stimulus in a periphery produces activity in corresponding location in the SC. Generates a motor command which turns the eye's fovea to the visual stimulus -> activates foveal region at the centre of SC
FixationHill of activity at SC's foveal region project to neurons in the brain stem and inhibit generation of saccades. Before the next saccade can begin, hill of activity at SC's foveal region must be removed a strong stimulus must appear at the peripheral SC or PER must disengage one's attention from the center and shift it to periphery. Fixation is an active process b/c not moving eyes.
Compare the 5 topographic areasretina, V1, PEF, FEF, SC. Location of a group of active cells indicate the relative location of a target in the visual field -> map for required size of movements. Contralateral stimulation, activity at centre maintains fixation, if you do both sides at the same time you get no movement.
V1 has large foveal representation as do other areas of the "what" stream. Retina, PEF, FEF, SC have large peripheral and small foveal representations, as do other areas in the "where" stream
While looking forward a visual stimulus appears at the peripheral left location but no motor response is requiredHill of action occurs in right retina and visual cortex (V1). Foveal activity in FEF (and SC) maintains forward fixation. Because attention is limited to forward direction the FEF and PEF are blind to stimulus A.
The same visual stimulus appears at left but now subject is required to saccade to leftHill of action occurs in right retina and visual cortex (V1) + enhanced activity in PEF, FEF, and SC. Activity in the right FEF initiated a saccade to the left stimulus
A visual stimulus appears at left and right, and the object is instructed to saccade to right and not leftleft and right retina + visual cortex activated, left PEF activated b/c attention and left FEF initate a saccade to the stimulus (from fixation where foveal activity in FEF)
A visual stimulus appears at left and the subject is required to make an arm movement to left while still fixating the centerRight retina + visual cortex activated, Foveal activity in FEF (SC) maintains forward fixation. Right PAF activated and Right PEF (attention selects a target for movement) activated leads to activation of approperiate limb motor areas.
Target shown in location A and one is asked to attend to it but not look at itactivity in foveal FEF maintains fixation. Activity in retina + V1, Activity is maintained in the PEF and in working memory in prefrontal cortex
A and B are briefly flashed in sequence and then both disappear, saccade to A then to B2 hills of activity in retina + V1, 2 hills of activity persist in PEF and Prefrontal cortex, Bilateral hill in the foveal area in FEF before sacchade initiated. After a saccade to the memory of A, A appears in foveal region of PEF and prefrontal cortex and B to a less peripheral location. Activity shifts from 1 set of neurons to another by a copy of saccadic eye movements known as corollary discharge. Originates in SC and directed to FEF (and to PF and PEF). Corollar discharge shifts the activity to group of neurons that would have been activated if A and B were still visible. Image lands where corollary discharge tells the PEF to expect them. The activity shifted by corollary discharge in PEF matches that arriving from visual cortex
Covert shiftsredirect attention to a spatial location without moving one's eyes, faster than saccades; selectively enhance the neural activity of corresponding retinotopic locations in early visual areas (V1). This in turn enhances the visual object's contrast, suppresses that of surrounding images, helps locate potential targets for a saccade to the object
which areas are involved in covert shifts in attention ?parietal (PEF) and prefrontal
cortical control of saccadessignals from V1 project along dorsal stream to PEF, FEF, and PF. PF exert positive influence on FEF, PEF, and V1 -> selectively tuning V1 to enhance one's perception of features that are relevant at the moment. Feedback process repeats as keep on examining patches of retina.
Is one conscious of covert and overt shifts in attention?No, each activates the shift hills of neural APs in PEF and PF as well as visual cortex. Can consciously direct our attention, conscious of whether we find or not find Waldo
Numerosity in IPSnumber of items in a group. Ability to compare numerosities present in young infants before they learnt to count and in a variety of animals incl. birds
Lesion in IPSimpairments in ability to perform spatial tasks, but also in understanding numbers or quantities and performing calculations
Numerosity topographic representation, small number rep where, large wherelocation of activity in IPS determines number size not the level of activity. In this map, small numbers produce activity in medial areas while large numbers in lateral areas. Easier to discriminate b/w small quantities than large quantities b/c larger area of neurons represent these smaller numbers
Frontal lobe numerosityfrontal lobe also involved in holding numerosity in working memory