The most sensitive inputs to cutaneous representing regions of primary somatosensory cortex do not change with behavioral training.

Learning a sensory detection task leads to an increased primary sensory cortex response to the detected stimulus, while learning a sensory discrimination task additionally leads to a decreased sensory cortex response to the distractor stimulus. Neural responses are scaled up, and down, in strength, along with concomitant changes in receptive field size. The present work considers neural response properties that are invariant to learning. Data are drawn from two animals that were trained to detect and discriminate spatially separate taps delivered to positions on the skin of their fingers. Each animal was implanted with electrodes positioned in area 3b, and responses were derived on a near daily basis over 84 days in animal 1 and 202 days in animal 2. Responses to taps delivered in the receptive field were quantitatively measured each day, and receptive fields were audiomanually mapped each day. In the subset of responses that had light cutaneous receptive fields, a preponderance of the days, the most sensitive region of the field was invariant to training. This skin region was present in the receptive field on all, or nearly all, occasions in which the receptive field was mapped, and this region constituted roughly half of the most sensitive region. These results suggest that maintaining the most sensitive inputs as dominant in cortical receptive fields provide a measure of stability that may be transformationally useful for minimizing reconstruction errors and perceptual constancy.

Task-dependent modulation of SI physiological responses to targets and distractors.

Selective attention experimental designs have shown that neural responses to stimuli in primary somatosensory cortex are stronger when the sensory stimuli are task relevant. Other studies have used animals under no task demands for data collection. The relationship between neural responses in the brain during behavior, and while an animal has no task demands, remains underexplored. We trained two animals to perform somatosensory detection for several weeks, followed by somatosensory discrimination for several weeks. Data in response to physically identical stimuli were collected from cortical implants while the animal was under no task demands before each behavioral session and also during that behavioral session. The Fourier spectra of the field potentials during detection or discrimination compared with the no task condition demonstrated suppression of the somatosensory µ-rhythm that is associated with readiness and anticipation of cognitive use of somatosensory and motor inputs. Responses to the task target were stronger during detection and discrimination than in the no task condition. The amplitude normalized time course of the target evoked response was similar in both cases. Evoked responses to the task distractor were not significantly stronger during behavior than in recordings under no task demands. The normalized time course of the distractor responses showed a suppression that peaks 30-35 ms after the onset of the response. The selectivity of this within trial suppression is the same as the selectivity of enduring suppression evident in studies of sensory cortical plasticity, which suggests the same neural process may be responsible for both.

Different neuroplasticity for task targets and distractors.

Adult learning-induced sensory cortex plasticity results in enhanced action potential rates in neurons that have the most relevant information for the task, or those that respond strongly to one sensory stimulus but weakly to its comparison stimulus. Current theories suggest this plasticity is caused when target stimulus evoked activity is enhanced by reward signals from neuromodulatory nuclei. Prior work has found evidence suggestive of nonselective enhancement of neural responses, and suppression of responses to task distractors, but the differences in these effects between detection and discrimination have not been directly tested. Using cortical implants, we defined physiological responses in macaque somatosensory cortex during serial, matched, detection and discrimination tasks. Nonselective increases in neural responsiveness were observed during detection learning. Suppression of responses to task distractors was observed during discrimination learning, and this suppression was specific to cortical locations that sampled responses to the task distractor before learning. Changes in receptive field size were measured as the area of skin that had a significant response to a constant magnitude stimulus, and these areal changes paralleled changes in responsiveness. From before detection learning until after discrimination learning, the enduring changes were selective suppression of cortical locations responsive to task distractors, and nonselective enhancement of responsiveness at cortical locations selective for target and control skin sites. A comparison of observations in prior studies with the observed plasticity effects suggests that the non-selective response enhancement and selective suppression suffice to explain known plasticity phenomena in simple spatial tasks. This work suggests that differential responsiveness to task targets and distractors in primary sensory cortex for a simple spatial detection and discrimination task arise from nonselective increases in response over a broad cortical locus that includes the representation of the task target, and selective suppression of responses to the task distractor within this locus.