Conference Agenda

A multisensory representation of the space close to the body is essential for interacting with the outside world. Traditional single neuron studies in macaques indicate that the ventral premotor cortex (PMv) plays a key role in building this representation by hosting the so-called peripersonal neurons (PPNs), which respond both to stimuli applied to the skin of different body parts and when they are moved in their proximity. However, classical studies employed single neuron recordings in passive and highly constrained conditions, leaving unclear whether the inferred spatial representation also applies to naturalistic conditions where the monkey can actively approach or avoid stimuli. Here, we adopted unbiased, chronic multi-electrode recordings and systematic stimulation of the body surface and of the peripersonal visual space to 1) construct the visual and tactile place fields of the recorded neurons in constrained conditions, to identify PPNs and 2) evaluate PPNs response in freely moving conditions. In this latter the monkey was left free to walk and climb in a 2x2x1.8 m transparent enclosure for neuroethological recording (NeuroEthoRoom - NER), and the same type of visual stimulations applied in constrained conditions was used in the NER. We recorded 142 neurons from two monkeys in both conditions. In the constrained condition, several neurons displayed clear visual or tactile place fields, whereas others fulfilled the definition of PPNs: tactile place fields were mostly contralateral and sometimes dependent on the stimulation direction, in line with the existing literature. Interestingly, the same neurons’ activity in the freely moving conditions revealed different types of relationship with the PPNs properties recorded in the classical restrained condition. These results represent the first attempt to investigate the flexibility of peripersonal space coding during unconstrained interaction of an animal with its environment.

Sensory events occurring close to the body have particular behavioural relevance. Perhaps unsurprisingly, these events elicit enhanced behavioural or neurophysiological responses. Such “peripersonal space” (PPS) responses, traditionally interpreted as reflecting the position of the stimulus in egocentric coordinates, may in fact play a role in creating or avoiding contact with objects near the body.

Here we provide a comprehensive characterisation of PPS responses across humans and non-human primates (NHP), with the aim of facilitating the assessment of their role in contact-related actions.

We recorded high-density EEG in humans and intracranial neuronal signals in NHPs during the same task. Visual and tactile stimuli were presented along a mediolateral axis centred on the right hand (distance from the hand: ±24, ±12, 0cm [visual]; 0cm [tactile]). Crucially, eye and hand positions were manipulated to disentangle eye- versus hand-centered encoding. Both scalp and intracortical local field potentials were analysed in the frequency domain by computing power spectral densities.

Preliminary results (24 humans, 1 NHP) revealed gaze-independent coding of stimulus position across species. In humans, alpha, beta, and gamma band power in posterior, lateralized regions was modulated by stimulus position. A qualitatively comparable positional coding was observed in NHP ventral premotor cortex. Further, hand position affected alpha and beta power in left-lateralised frontal and posterior electrodes, though this effect was rather independent of stimulus distance.

Additional computational modelling analyses assess whether these responses represent action intention. Results indicate that, as well as reflecting the distance between the stimulus and an effector, the neurophysiological correlates of PPS representation reflect the biomechanical properties of potential contact-actions.

Our data provide the first cross-species comparable characterisation of the neurophysiological correlates of PPS representation. Further, the results support the notion that PPS representation also reflects the value of actions which create or avoid contact with the body.

Peripersonal space (PPS) is a highly plastic “invisible bubble” surrounding the body, whose boundaries are coded through the integration of multisensory stimuli arising from the body and the nearby space. Multisensory integration preferentially occurs when visual inputs coming from the environment are simultaneously presented close to the body district from which somatosensory signals originate, within the PPS. Hence, multisensory integration can be considered a proxy of an effective coding of the self-body boundaries. In a series of experiments, we focused on the hand-centered PPS to explore whether and how the proximity to another-hand plastically shapes the PPS size. To this aim, we recorded behavioural and electrophysiological responses to visuo-tactile stimuli in both normal and pathological contexts.

When the self-other discrimination mechanism is normally working, we demonstrated that the spatial proximity to someone else’s hand shrinks the portion of space within which multisensory responses occur, thus reducing the PPS boundaries. This suggests that PPS representation, built from bodily and multisensory signals, plastically adapts to the presence of others to define self-other boundaries, so that what is usually coded as ‘‘my space’’ is recoded as ‘‘your space’’.

When the self-other discrimination mechanism is altered due to brain damage, so that patients claim that another hand is their own (Pathological Embodiment), we demonstrated a body-ownership dependent modulation of PPS boundaries. Indeed, multisensory responses occur when visual stimuli appear close to the other-hand believed to be one’s own, thus revealing a shift of the PPS representation towards the portion of space surrounding the other-hand, so that what is normally coded as ‘‘your space’’ is recoded as ‘‘my space’’. This supports the view that PPS representation is built upon the belief about the body, suggesting that delusional body-ownership leads to code as self-space the space surrounding the hand believed to be one’s own.

The superior colliculus (SC) is a midbrain structure receiving direct projections from the retina and hosting topographic sensory and motor maps to guide spatially-oriented behaviors. In monkeys, the SC’s topographic organization has been revealed using electrophysiology, mainly focusing on its motor properties. In humans, SC’s retinotopic maps have been obtained with fMRI, but leaving poorly documented the representation of the peripheral visual field due to experimental constrains. Here, we aimed to fill this gap between SC’s visual topography in human and monkey by investigating the retinotopic organization of the SC in macaques with wide-field stimuli and fMRI-based retinotopic mapping.

We first performed a phase-encoded retinotopic mapping experiment (3T, 1.25mm isotropic voxels) in two monkeys, using clockwise-counterclockwise rotating wedges and expanding-contracting rings covering 80° of the visual field. The apertures were superimposed on two types of high-contrast, dynamic, colorful, multi-objects textures. In separate block-design fMRI experiments, we also presented spatially restricted stimuli of different sizes (18° and 25° diameter) at different eccentricities (center, 12.5° and 16.5° in the lower left/right quadrants), consisting of either a static ovaloid aperture superimposed on one texture previously used (colorful objects of different category-types, updated at 6 fps) or single categories of monkey faces, monkey bodies, objects, and scrambles.

The eccentricity and polar angle maps revealed a clear contralateral representation of the visual field in the SC, with features consistent with previously reported maps based on fMRI (humans) and electrophysiology (monkey). The fovea is represented rostrally and the periphery caudally, upper and lower quadrants medio-laterally. The response to spatially-restricted stimuli confirmed this topographic organization. Interestingly, the representation of the less eccentric regions differed between left and right SC, and this cannot be accounted for by differences in tSNR.

These findings provide the first evidence of wide-field retinotopic maps in the macaque SC using fMRI.