A review article by Suthana and Fried (citation below) has just been published on the nature of medial temporal lobe neurons. It is very informative.
The medial temporal lobe (MTL) includes the hippocampus, entorrhinal cortex, perirhinal cortex, parahippochampal cortex, and amygdala. It is associated with memory but not perception.
The ability to form new episodic memories, which can later be consciously accessed, relies on an intact hippocampus and surrounding MTL. However, other functions, such as visual perception, do not depend on an intact MTL . …The MTL receives multimodality sensory input from wide areas of the cortex, thereby offering the possibility of convergence and integration of information. Within the MTL, the hippocampus is positioned at the top of a multisensory hierarchy, receiving converging incoming sensory information through the entorhinal cortex that is either object identifying (via perirhinal cortex) or spatially informative (via parahippocampal cortex). For the MTL to encode a given episode in time properly, the event must be first perceived and processed in upstream sensory cortices.
The response of MTL neurons has been recorded and characterized.
(i) Responses are selective. MTL neurons can respond to particular stimulus categories (e.g. faces, outdoor scenes, animals, etc.) or to individual stimuli (e.g. a family member, a famous individual, or a particular landmark) during the passive viewing of visual stimuli.
(ii) Responses are invariant. An MTL neuron may respond to a particular stimulus, such as a particular face or landmark, but in addition it will also respond to other stimuli representing that particular face or landmark, even if these stimuli are distinctly different in stimulus features compared with the original stimulus. Thus, a neuron may respond to a picture of the Sydney Opera House and exhibit no response to 50 other landmarks, yet also respond to many permutations and physically different representations of the Sydney Opera House, seen in color, in black and white, or from different angles. In fact, the neuron may also respond to the iconic representation, namely the words ‘Sydney Opera’, which is obviously different in its visual properties compared with the image of this landmark. Recently, it was shown that this invariance crosses modalities, meaning that MTL neurons may exhibit a selective and ‘invariant’ response to a particular stimulus out of 100 images and do so independently of the sensory modality (visual image, audio, or written iconic representations) through which the stimulus was presented. Results were consistent with the anatomical hierarchy within the MTL; the highest percentage of neurons with modality-independent invariant responses was found in the hippocampus and entorhinal cortex, compared with the amygdala or parahippocampal cortex.
(iii) Responses are late. The selective and invariant responses described above are of relative long latencies, often in the 300–500 ms range. Interestingly and consistent with the anatomical hierarchy, responses in the parahippocampal cortex are significantly shorter than those in the hippocampus, amygdala, and entorhinal cortex, but still with longer latencies than those observed in animal studies.
(iv) Responses are associated with conscious perception. Using flash suppression and backward masking paradigms, it has been shown that selective MTL responses are mainly triggered when accompanied by the participants’ conscious perception or recognition of the stimulus. When varying the display time of an image between 33 and 256 m, MTL neurons selective for an image significantly increase in firing rate only if the image is recognized by the subject, even if the image is presented for as briefly as 33 ms. Conversely, if the subject reported no recognition of the image, the MTL neuron selective for that image is mostly silent.
Responses can be internally generated. The act of re-experiencing a previous episode can be internally generated or cued by an external percept within the environment. Generating an internal percept of a stimulus through imagery in the absence of an external visual stimulus, recruits the same MTL neuron activated during viewing of the stimulus itself. Subjects’ ability to modulate these neurons was studied … Subjects were told to try and control which of two competing images would be projected on an external display; the displayed images were controlled by the firing rate of recorded MTL neurons selective for the two images. Subjects were able to control successfully which image was projected by altering the firing rate of two independently selective neurons independent of the visual input of the stimulus on the screen. These results highlight the power of internal representation to override sensory input. They also illustrate how human single neuron recordings can illuminate mechanisms of conscious perception, regardless of whether they are externally or internally generated.
This does appear to be the sort of response that could give us episodic memory.
Why are these responses present in the MTL, and in the hippocampus and entorhinal cortex in particular? Given the critical role of these regions in episodic memory, we posit that these responses are central in the transformation of novel stimuli to representations that can be later consciously retrieved as episodic memories. As such, these representations need to have detail but also abstraction (i.e. the loss of detail), so that they can be later summoned by internal as well as external cues. It is also possible that consciously perceived familiar stimuli trigger the recollection of an associated memory and, thus, reactivate MTL neurons. … Although patients with damage to the MTL can no longer form new episodic memories, their visual perceptual function remains intact. The correlation of single neuron responses in the MTL with specific conscious percepts does not imply that these regions are necessary for conscious percepts, yet these responses may reflect the link between conscious percepts and episodic memories that can be later
consciously accessed. … Strikingly, hippocampal and entorhinal neurons that were selectively active during the initial viewing of distinct episodes were similarly reactivated just before the verbal report of recall of those very same episodes. Although sustained selective responses during viewing of episodes was present in neurons of other brain regions, such as amygdala and frontal cortex, the specific reactivation before reported recall was only present in the hippocampus and entorhinal cortex.
Interestingly the synchrony of waves may imply a that memory and consciousness are working together.
Human intracranial studies have shown that the spiking rate of single hippocampal neurons predicts whether a recently learned item will be remembered. In addition, intracranial recording of local field potentials (LFPs) have yielded important insights. Theta LFP oscillations (3–8 Hz) have been widely implicated in human memory and the strength of their amplitude measured in the MTL has been shown to predict the success of episodic encoding in humans. Interestingly, the theta amplitude that predicted recall success was also strongly linked to the gamma oscillation (30– 100 Hz). The phase of theta oscillations and their relationship to gamma oscillations in monkeys and humans have been related to memory performance. … It has been hypothesized that the theta- spiking relation may reflect the cued recall of an upcoming item stored in memory. A recent study in humans showed that the relation between spiking and theta during encoding predicted memory success. … A tighter coordination between hippocampal single neuron spiking and the simultaneously recorded theta LFP oscillation during initial viewing of the image predicted the success of the formed memory for that image . These results implicate a direct role for theta-linked spiking activity in episodic memory.
Nanthia Suthana, & Itzhak Fried (2012). Percepts to recollections: insights from single neuron recordings in the human brain Trends in Cognitive Sciences, 16 (8) DOI: 10.1016/j.tics.2012.06.006