We discussed a few different methods by which neuroscientists can attempt to est

Question

We discussed a few different methods by which neuroscientists can attempt to establish causal links between neural activity and perception (electrical microstimulation, reversible chemical inactivation, surgical lesions). Consider the following questions about these approaches: a) What key property of a neural representation of sensory stimuli is required in order to be able to make sensible use of electrical microstimulation? Why is this property important for microstimulation effects to be interpretable? (10 points) b) Permanent lesions (both experimentally-induced and naturally-occurring) have been a valuable tool in the history of neuroscience. Consider the results of the Newsome and Pare (1988) study that we discussed in class. How do you interpret the finding of Newome and Pare that the direction discrimination performance of monkeys recovered substantially within 3 weeks following the surgically-placed lesion? What factor(s) might account for the recovery? (10 points) c) You are interested in determining whether primary visual cortex (V1) in the mouse makes a causal contribution to perception of stimulus orientation. So you train mice to report whether the orientation of a bar of light is clockwise (CW) or counter-clockwise (CCW) relative to vertical (a task similar to what we discussed in the lecture on behavioral methods). Once you have trained the mice, you decide to implement a causal test of whether mouse V1 neurons contribute to this behavior. As you are researching your approach, you discover recent literature which shows that there are no orientation columns in mouse V1 (unlike in cats and monkeys). Rather, nearby neurons in mouse V1 have orientation preferences that are randomly related. However, you also learn that mouse V1 does contain an orderly retinotopic map of visual space, such that nearby neurons have similar receptive field locations. You have 3 methods available to you to conduct your experiment: 1) electrical microstimulation, 2) reversible inactivation using localized injections of muscimol into V1 (muscimol is a GABA-A agonist that silences neural activity within minutes of injection into the brain; after several hours the effect wears off), and 3) permanent lesions of all or part of V1. Which method would you choose and why would it be the best choice for understanding the normal function of V1? Explain your choice clearly and logically. (20 points)

Explanation / Answer

a) What key property of a neural representation of sensory stimuli is required in order to be able to make sensible use of electrical microstimulation?

ans:In light of the discoveries of the recent past, two factors are accountable for the determination of the spatial extent of neural activation by microstimulation, namely viz:

(i) the physical spread of current, with the rate of decrement of current strength is proportional to the square of the distance from the electrode tip.

and;

(ii)the excitability property of the neural elements within the range of the volume, where myelinated axons and the initial segment are most excitable.

#Why is this property important for microstimulation effects to be interpretable?

ans:Microstimulation effects on spatial extent of neural activation is depicted by clustered representations of sensory space. This is readily interpretable by the experimental subject, because it more closely mimics the natural sensory activation of neurons.

#How do you interpret the finding of Newome and Pare that the direction discrimination performance of monkeys recovered substantially within 3 weeks following the surgically-placed lesion?What factor(s) might account for the recovery?

ans:Newsome and Pare tested for alterations in the activity of middle temporal (MT) neurons.They concluded that these alterations were sufficient to change an animal's perception of visual motion using a rigorous psychophysical measure of perceptual performance. They trained monkeys to discriminate the motion of dots moving within an aperture. The motion was adjusted in terms of directional coherence among the dots,ranging from 100 %(coherent) to 0 per cent ( random). Explanation: The MT neurons are organized into columns in which the cells in vicinity are selective in nature for the same direction of motion. This feature allows the experimenter to stimulate neurons which are tuned in the same manner. The monkeys' perception towards the preferred direction of neurons was biased due microstimulation of MT in the stimulated column when the motion stimulus was weakly coherent. The consistent effect of stimulation on motion perception irrespective of the veridical visual stimulus underlines the fact activity that were stimulation driven and visually driven was combined in an additive manner. A recent finding is of the opinion that neural signals produced by microstimulation are filtered according to behavioural significance.This study mimics the information that enters the visual system through the eye.

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