Plastic reorganizations of the receptive fields of neurons of the auditory cortex and the medial geniculate body induced by microstimulation of the auditory cortex

Receptive field plasticity of area 17 visual cortical neurons of adult rats

In contrast to somatosensory cortex (SI), where the pervasiveness of reorganizational capacities is well-established, plasticity of receptive fields (RFs) of adult primary visual cortex (VI) remains controversial. To investigate RF plasticity in VI of adult rats, we here used intracortical microstimulation (ICMS) to overcome particularities related to stimulus presentation and training procedures which limit comparison across modalities. Our results show that VI RFs can be altered by ICMS; however, changes depended on the pre-ICMS RF size. Initially small RFs expanded after 2 h of ICMS with little signs of recovery within the next hours, while initially large RFs remained unaffected. Inspection of the time course of neuron responses revealed, however, that in large RFs early response components were enhanced, while late response components were reduced resulting in changes of the spatiotemporal RF properties. Although plastic changes in VI showed a substantial heterogeneity, our results indicate a capacity of VI neurons to undergo plastic changes comparable to SI neurons. However, the magnitude and aspects of reversibility appeared to be different suggesting a significant modality-specificity of reorganizational changes of cortical sensory neurons.

Intracortical microstimulation induced changes in spectral and temporal response properties in cat auditory cortex

Intracortical microstimulation (ICMS), consisting of a 40 ms burst (rate 300 Hz) of 10 microA pulses, repetitively administered once per second, for a total duration of 1 h, induced cortical reorganization in the primary auditory cortical field of the anesthetized cat. Multiple single-unit activity was simultaneously recorded from three to nine microelectrodes. Spiking activity was recorded from the same units prior to and following the application of ICMS in conjunction with tone pips at the characteristic frequency (CF) of the stimulus electrode. ICMS produced a significant increase in the mean firing rate, and in the occurrence of burst activity. There was an increase in the cross-correlation coefficient (R) for unit pairs recorded from sites distant from the ICMS site, and a decrease in R for unit pairs that were recorded at the stimulation site. ICMS induced a shift in the CF, dependent on the difference between the baseline CF and the ICMS-paired tone pip frequency. ICMS also resulted in broader tuning curves, increased driven peak firing rate and reduced response latency. This suggests a lasting reduction in inhibition in a small region surrounding the ICMS site that allows expansion of the frequency range normally represented in the vicinity of the stimulation electrode.

Plasticity of orientation preference maps in the visual cortex of adult cats

In contrast to the high degree of experience-dependent plasticity usually exhibited by cortical representational maps, a number of experiments performed in visual cortex suggest that the basic layout of orientation preference maps is only barely susceptible to activity-dependent modifications. In fact, most of what we know about activity-dependent plasticity in adults comes from experiments in somatosensory, auditory, or motor cortex. Applying a stimulation protocol that has been proven highly effective in other cortical areas, we demonstrate here that enforced synchronous cortical activity induces major changes of orientation preference maps (OPMs) in adult cats. Combining optical imaging of intrinsic signals and electrophysiological single-cell recordings, we show that a few hours of intracortical microstimulation (ICMS) lead to an enlargement of the cortical representational zone at the ICMS site and an extensive restructuring of the entire OPM layout up to several millimeters away, paralleled by dramatic changes of pinwheel numbers and locations. At the single-cell level, we found that the preferred orientation was shifted toward the orientation of the ICMS site over a region of up to 4 mm. Our results show that manipulating the synchronicity of cortical activity locally without invoking training, attention, or reinforcement, OPMs undergo large-scale reorganization reminiscent of plastic changes observed for nonvisual cortical maps. However, changes were much more widespread and enduring. Such large-scale restructuring of the visual cortical networks indicates a substantial capability for activity-dependent plasticity of adult visual cortex and may provide the basis for cognitive learning processes.

Tonotopic cortical changes following stapes substitution in otosclerotic patients: a magnetoencephalographic study

The aim of the study was to investigate and follow up the tonotopic organization of the primary auditory cortex in otosclerotic patients before and after corrective surgery. The characteristics of primary auditory cortex activation were studied in ten otosclerotic patients (i.e., subjects suffering from a conductive hearing loss, prior to and following stapes substitution). Magnetoencephalographic recordings of auditory evoked fields by tone-burst stimulation at octave frequencies between 250 and 2000 Hz were performed during monaural stimulation. The brain topography of the main cortical response (N100m) generators at different tones was studied in patients and compared with ten healthy controls; pre- post-surgical changes were also correlated to their clinical outcome following corrective surgery. A significant decrease of the tonotopic extension in the cortical region responsive to the four explored frequencies was found in patients before surgery with respect to the control population. At the time of postsurgical follow-up, the tonotopic representation had enlarged and was approaching the dimensions seen in normal subjects, although with higher variability. The extent of the enlargement of the postoperative tonotopically organized area was directly correlated with the postsurgery period duration. Our findings indicate that auditory cortical areas of human adults undergo functional reorganization following peripheral alteration of the sensory input entering the CNS. The restriction of the cortical tonotopic region caused by the long-term reduction of acoustic input is followed by a reorganization within the usual boundaries following the recovery of auditory function; this process is taking place in a time scale of a few weeks.