Both hearing and touch are sensitive to the frequency of mechanical oscillations-sound waves and tactile vibrations, respectively. The mounting evidence of parallels in temporal frequency processing between the two sensory systems led us to directly address the question of perceptual frequency equivalence between touch and hearing using stimuli of simple and more complex temporal features. In a cross-modal psychophysical paradigm, subjects compared the perceived frequency of pulsatile mechanical vibrations to that elicited by pulsatile acoustic (click) trains, and vice versa. Non-invasive pulsatile stimulation designed to excite a fixed population of afferents was used to induce desired temporal spike trains at frequencies spanning flutter up to vibratory hum (>50 Hz). The cross-modal perceived frequency for regular test pulse trains of either modality was a close match to the presented stimulus physical frequency up to 100 Hz. We then tested whether the recently discovered "burst gap" temporal code for frequency, that is shared by the two senses, renders an equivalent cross-modal frequency perception. When subjects compared trains comprising pairs of pulses (bursts) in one modality against regular trains in the other, the cross-sensory equivalent perceptual frequency best corresponded to the silent interval between the successive bursts in both auditory and tactile test stimuli. These findings suggest that identical acoustic and vibrotactile pulse trains, regardless of pattern, elicit equivalent frequencies, and imply analogous temporal frequency computation strategies in both modalities. This perceptual correspondence raises the possibility of employing a cross-modal comparison as a robust standard to overcome the prevailing methodological limitations in psychophysical investigations and strongly encourages cross-modal approaches for transmitting sensory information such as translating pitch into a similar pattern of vibration on the skin.

The perceived intensity of a vibrotactile stimulus is thought to depend on single-neuron firing rates (rate coding) and the number of active afferents (population coding). Unaddressed until now is whether the temporal relation of individual spikes also conveys information about tactile intensity. We used cutaneous electro-tactile stimulation to investigate how the temporal structure of a fixed number of spikes in a 1-s train influenced the perception of intensity. Four mean spike rates spanning the flutter and vibra-tory hum range (36 Hz, 60 Hz; 120 Hz, 180 Hz) were tested, with spikes grouped into a regular pattern, or bursts of 2-6 spikes spaced 3 ms apart. To link a putative neural code to perception, perceived intensity was assessed in 16 human participants (aged 20-45; 4 females) using the psychophysical paradigm of magnitude estimation. Compound sensory nerve action poten-tials were recorded to assess any stimulus variation in afferent recruitment. The temporal structuring of a fixed number of spikes into periodic bursts of multiple spikes altered perceived intensity as a function of burst spike count. The largest increase was seen at 36 Hz, where the bursts of six spikes were rated 2.1 times stronger than the regularly spaced spikes [95% confidence interval (CI): 1.9-2.3]. The true increase is likely larger as temporal structuring of spikes into bursts had some negative effect on afferent recruitment. We conclude that the perceived intensity can be modulated by changing temporal features of afferent dis-charge even when normalized for the number of recruited afferents.NEW & NOTEWORTHY Structuring a fixed number of spikes into temporal burst patterns evoke gradations of perceived intensity with burst spike count, emphasizing the importance of spike timing in primary afferents for shaping perception. This forms the basis for new strategies in communicating a range of intensity information to users of neural interfaces by simply varying the tim-ing of spikes in nonspecific primary afferents using fixed-charge electric pulses, without requiring alterations in stimulation cur-rent or mean pulse frequency.

When tactile afferents were manipulated to fire in periodic bursts of spikes, we discovered that the perceived pitch corresponded to the inter-burst interval (burst gap) in a spike train, rather than the spike rate or burst periodicity as previously thought. Given that tactile frequency mechanisms have many analogies to audition, and indications that temporal frequency channels are linked across the two modalities, we investigated whether there is burst gap temporal encoding in the auditory system. To link this putative neural code to perception, human subjects (n = 13, 6 females) assessed pitch elicited by trains of temporally-structured acoustic pulses in psychophysical experiments. Each pulse was designed to excite a fixed population of cochlear neurons, precluding place of excitation cues, and to elicit desired temporal spike trains in activated afferents. We tested periodicities up to 150 Hz using a variety of burst patterns and found striking deviations from periodicity-predicted pitch. Like the tactile system, the duration of the silent gap between successive bursts of neural activity best predicted perceived pitch, emphasising the role of peripheral temporal coding in shaping pitch. This suggests that temporal patterning of stimulus pulses in cochlear implant users might improve pitch perception. © 2022, The Author(s).