Spatially selective firing of place cells, grid cells, boundary vector/border cells and head direction cells constitutes the basic building blocks of a canonical spatial navigation system centered on the hippocampal-entorhinal complex. While head direction cells can be found throughout the brain, spatial tuning outside the hippocampal formation is often non-specific or conjunctive to other representations such as a reward. Although the precise mechanism of spatially selective firing activity is not understood, various studies show sensory inputs, particularly vision, heavily modulate spatial representation in the hippocampal-entorhinal circuit. To better understand the contribution of other sensory inputs in shaping spatial representation in the brain, we performed recording from the primary somatosensory cortex in foraging rats. To our surprise, we were able to detect the full complement of spatially selective firing patterns similar to that reported in the hippocampal-entorhinal network, namely, place cells, head direction cells, boundary vector/border cells, grid cells and conjunctive cells, in the somatosensory cortex. These newly identified somatosensory spatial cells form a spatial map outside the hippocampal formation and support the hypothesis that location information modulates body representation in the somatosensory cortex. Our findings provide transformative insights into our understanding of how spatial information is processed and integrated in the brain, as well as functional operations of the somatosensory cortex in the context of rehabilitation with brain-machine interfaces.

9Spatially selective firing in the forms of place cells, grid cells, border cells and head 10 direction cells are basic building blocks of a canonical spatial circuit centered on the 11 hippocampal-entorhinal complex. While head direction cells can be found throughout 12 the brain, spatial tuning outside of the parahippocampal regions are often non-specific 13 or conjunctive to other representations such as a reward. While the precise mechanism 14 of spatially selective activities is not understood, various studies show sensory inputs 15 (particularly vision) heavily modulate spatial representation in the hippocampal-16 entorhinal circuit. To better understand the contribution from other sensory inputs in 17 shaping spatial representation in the brain, we recorded from the hindlimb region of the 18 primary somatosensory cortex (S1HL) in foraging rats. To our surprise, we were able 19 to identify the full complement of spatial activity patterns reported in the hippocampal-20 entorhinal circuit, namely, place cells, head direction cells, boundary vector/border cells, 21 grid cells and conjunctive cells. This novel finding supports the hypothesis that location 22 information is necessary for body representation in the S1, and may be analogous to 23 spatially tuned representations in the motor cortex relating to the movement of body 24 parts. Our findings are transformative in our understanding of how spatial information 25 is used and utilized in the brain, as well as functional operations of the S1 in the context 26 of rehabilitation with brain-machine interfaces. 27 3 / 55 109 6 / 55neurons (24.7%) would be classified as place cells (average SI of 1.67 ± 0.04; cut-off 110 = .88). This percentage was substantially higher than expected with random selection 111 from the entire shuffled population ( Fig. 1D; Z = 33.64, P < 0.001; binomial test with 112 expected P0 of 0.01 among large samples). In comparison to previous reports in the 113 hippocampus, there appeared to be a lower percentage of place cells in the S1HL 114 (24.7%) compared to the hippocampus CA1 (15/24, (Langston et al., 2010)), but SI 115 scores appeared to be higher (1.67 vs <1.2) in the S1HL. We also reported that average 116 peak rate (S1HL: 8.72 ± 0.70 Hz vs hippocampus: 7.92 ± 0.56 Hz), mean firing rate 117 (S1HL: 0.49 ± 0.04 Hz vs hippocampus: 0.64 ± 0.1 Hz), field size ( Fig. 1F; S1HL: 118 733.31 ± 42.60 cm 2 vs hippocampus: 619 ± 116 cm 2 ), average spatial coherence (Fig. 119 1G ; S1HL: 0.61 ± 0.02 vs hippocampus: ~0.6), spatial sparsity ( Fig. 1H; S1HL: 0.11 120 ± 0.01 vs hippocampus: 0.32 ± 0.02) and proportion of multi-field place cells (S1HL: 121 29.3% vs hippocampus: 22.2%) were all comparable to hippocampal place cells (Fyhn 122 et al., 2004; Jung et al., 1994; Kjelstrup et al., 2008; McNaughton et al., 1983; Skaggs 123 et al., 1996; Wills et al., 2010). Besides, S1HL place cells can be found at any position 124 within the arena (Fig. 1E) as in the hippocampus (Rich et al., 2014). 125Place cell activity was defined not on...

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