Insights into cortical mechanisms of behavior from microstimulation experiments

Abstract

Even the simplest behaviors depend on a large number of neurons that are distributed across many brain regions. Because electrical microstimulation can change the activity of localized subsets of neurons, it has provided valuable evidence that specific neurons contribute to particular behaviors. Here we review what has been learned about cortical function from behavioral studies using microstimulation in animals and humans. Experiments that examine how microstimulation affects the perception of stimuli have shown that the effects of microstimulation are usually highly specific and can be related to the stimuli preferred by neurons at the stimulated site. Experiments that ask subjects to detect cortical microstimulation in the absence of other stimuli have provided further insights. Although subjects typically can detect microstimulation of primary sensory or motor cortex, they are generally unable to detect stimulation of most of cortex without extensive practice. With practice, however, stimulation of any part of cortex can become detected. These training effects suggest that some patterns of cortical activity cannot be readily accessed to guide behavior, but that the adult brain retains enough plasticity to learn to process novel patterns of neuronal activity arising anywhere in cortex.

Fig. 1

Distributions of detection thresholds. Thresholds for detecting electrical microstimulation delivered by a microelectrode in visual cortex. (A) Representative psychometric detection function for a V1 site. Threshold (5 μA) was taken as the current yielding 82% correct, where 50% represents chance. Error bars are SEM. (B) Distributions of detection thresholds for sites in different visual areas. Triangles mark medians. There was relatively little variance in thresholds within and between visual areas. Reproduced with permission from Murphey and Maunsell (2007).

Fig. 2

Penfield stimulation sites. Effects from stimulating human cerebral cortex. This figure summarizes effects reported by Penfield and Rasmussen (1950) in testing many patients. Points of different color mark the approximate locations where different patient responses were evoked, as indicated by the key. The authors did not plot the locations of somatosensory reports, so those data are not shown. Most responses were obtained in or around primary sensory or motor cortex, and large regions of cortex did not reliably produce movements or percepts.

Fig. 3

V1 Microstimulation training. Effects of training to detect microstimulation of V1. (A) Threshold current needed for behavioral detection of electrical stimulation of a small V1 region as a function of time. The monkey initially could not detect 50 μA stimulation, but thresholds gradually improved and stabilized near 6 μA over the course of many days when sites in a 3 mm × 3 mm region of V1 were tested. (B) Training was local and long lasting. After training the site in A, electrical stimulation at a distant V1 site required retraining to achieve low thresholds. Once trained, the effects were permanent. Thresholds were stable for over a year without further training at this site. (C) Training to detect electrical stimulation at the site in A impaired detection of visual targets. When visual stimuli were placed at the corresponding retinal location, thresholds were far above the normal range (which is marked by the thin horizontal band), but gradually returned to normal when the animal practiced detecting those visual stimuli. (D) Retraining with visual stimuli at the site in A and C did not improve visual detection at other sites. Visual thresholds were elevated for the site in B (initially the animal could not see stimuli of 100% contrast), but also improved when the animal practiced with visual stimuli in that retinal location. The horizontal band marks the range of normal visual thresholds for this site. Reproduced with permission from Ni and Maunsell (2010).