Comparative population analysis of cortical representations in parametric spaces of visual field and skin: a unifying role for nonlinear interactions as a basis for active information processing across modalities

Differential effects of the temporal and spatial distribution of audiovisual stimuli on cross-modal spatial recalibration

Visual input constantly recalibrates auditory spatial representations. Exposure to isochronous audiovisual stimuli with a fixed spatial disparity typically results in a subsequent auditory localization bias (ventriloquism aftereffect, VAE), whereas exposure to spatially congruent audiovisual stimuli improves subsequent auditory localization (multisensory enhancement, ME). Here, we tested whether cross-modal recalibration is affected by the stimulation rate and/or the distribution of audiovisual spatial disparities during training. Auditory localization was tested before and after participants were exposed either to audiovisual stimuli with a constant spatial disparity of 13.5° (VAE) or to spatially congruent audiovisual stimulation (ME). In a between-subjects design, audiovisual stimuli were presented either at a low frequency of 2 Hz, as used in previous studies of VAE and ME, or intermittently at a high frequency of 10 Hz, which mimics long-term potentiation (LTP) protocols and which was found superior in eliciting unisensory perceptual learning. Compared to low-frequency stimulation, VAE was reduced after high-frequency stimulation, whereas ME occurred regardless of the stimulation protocol. In two additional groups, we manipulated the spatial distribution of audiovisual stimuli in the low-frequency condition. Stimuli were presented with varying audiovisual disparities centered around 13.5° (VAE) or 0° (ME). Both VAE and ME were equally strong compared to a fixed spatial relationship of 13.5° or 0°, respectively. Taken together, our results suggest (a) that VAE and ME represent partly dissociable forms of learning and (b) that auditory representations adjust to the overall stimulus statistics rather than to a specific audiovisual spatial relationship.

A Mechanical Stimulation Glove to Induce Hebbian Plasticity at the Fingertip

Repetitive sensory stimulation of the fingertip induces Hebbian plasticity in the sensorimotor cortex that benefits the tactile and motor behavior of the hand in healthy younger adults, older adults, and patients. To use this method outside the laboratory, robust and portable stimulation systems are needed that allow prolonged stimulation phases over several hours without compromising on signal intensity or personal mobility. Here, we introduce two stimulation gloves that apply finger- and frequency-specific mechanical stimulation to individual fingertips over prolonged periods. The stimulators are built into commercially available cotton gloves and apply stimulation either via loudspeaker membranes or via linear resonant actuators (LRAs). We tested the efficiency of both gloves to induce Hebbian plasticity in younger adults by using two established measures of tactile performance, the grating orientation task (GOT), and the two-point discrimination task (2PDT). Both tests were performed before and after 3 h of sensory finger stimulation using one of either glove system. As a control condition, a non-stimulated finger was tested in both tasks before and after stimulation. The results show no significant effect of sensory stimulation on GOT thresholds, but a significant decrease in the 2PDT thresholds after compared to before the training at the stimulated finger only. The loudspeaker membrane improved performance in the 2PDT in 10/16 participants, whereas the LRA improved performance in the 2PDT in 13/16 participants. Stimulation gloves with built-in modules may be used in future larger-scale cohort studies on sensorimotor plasticity, rehabilitation, and learning.

Tactile acuity charts: a reliable measure of spatial acuity

For assessing tactile spatial resolution it has recently been recommended to use tactile acuity charts which follow the design principles of the Snellen letter charts for visual acuity and involve active touch. However, it is currently unknown whether acuity thresholds obtained with this newly developed psychophysical procedure are in accordance with established measures of tactile acuity that involve passive contact with fixed duration and control of contact force. Here we directly compared tactile acuity thresholds obtained with the acuity charts to traditional two-point and grating orientation thresholds in a group of young healthy adults. For this purpose, two types of charts, using either Braille-like dot patterns or embossed Landolt rings with different orientations, were adapted from previous studies. Measurements with the two types of charts were equivalent, but generally more reliable with the dot pattern chart. A comparison with the two-point and grating orientation task data showed that the test-retest reliability of the acuity chart measurements after one week was superior to that of the passive methods. Individual thresholds obtained with the acuity charts agreed reasonably with the grating orientation threshold, but less so with the two-point threshold that yielded relatively distinct acuity estimates compared to the other methods. This potentially considerable amount of mismatch between different measures of tactile acuity suggests that tactile spatial resolution is a complex entity that should ideally be measured with different methods in parallel. The simple test procedure and high reliability of the acuity charts makes them a promising complement and alternative to the traditional two-point and grating orientation thresholds.

Cortical topography of intracortical inhibition influences the speed of decision making

The neocortex contains orderly topographic maps; however, their functional role remains controversial. Theoretical studies have suggested a role in minimizing computational costs, whereas empirical studies have focused on spatial localization. Using a tactile multiple-choice reaction time (RT) task before and after the induction of perceptual learning through repetitive sensory stimulation, we extend the framework of cortical topographies by demonstrating that the topographic arrangement of intracortical inhibition contributes to the speed of human perceptual decision-making processes. RTs differ among fingers, displaying an inverted U-shaped function. Simulations using neural fields show the inverted U-shaped RT distribution as an emergent consequence of lateral inhibition. Weakening inhibition through learning shortens RTs, which is modeled through topographically reorganized inhibition. Whereas changes in decision making are often regarded as an outcome of higher cortical areas, our data show that the spatial layout of interaction processes within representational maps contributes to selection and decision-making processes.

Shorter latencies for motion trajectories than for flashes in population responses of cat primary visual cortex

Psychophysical evidence in humans indicates that localization is different for stationary flashed and coherently moving objects. To address how the primary visual cortex represents object position we used a population approach that pools spiking activity of many neurones in cat area 17. In response to flashed stationary squares (0.4 deg) we obtained localized activity distributions in visual field coordinates, which we referred to as profiles across a 'population receptive field' (PRF). We here show how motion trajectories can be derived from activity across the PRF and how the representation of moving and flashed stimuli differs in position. We found that motion was represented by peaks of population activity that followed the stimulus with a speed-dependent lag. However, time-to-peak latencies were shorter by approximately 16 ms compared to the population responses to stationary flashes. In addition, motion representation showed a directional bias, as latencies were more reduced for peripheral-to-central motion compared to the opposite direction. We suggest that a moving stimulus provides 'preactivation' that allows more rapid processing than for a single flash event.