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Single-neuron responsesSingle-neuron responsesHuman single-neuron responses at the threshold of recognition R. Quiroga, R. Malach et al, University of Leicester, Weizmann Institute PNAS, March 4 2008, vol. 5, no. 9, pp. 3599-604/doi/10.1073/ http://pnas.org/ INTRODUCTION: This study serves to refute one of the popular arguments of twentieth century consciousness studies to the effect that consciousness was ‘just what it was like to have a brain or neural processing’. The study demonstrates that the exactly same signal, with a duration that placed it on the boundary of being consciously recognised or not recognised, produced almost no response, if it was not consciously recognised, but a vigorous response, if it was consciously recognised. A further point of interest, not discussed in the study itself, is the relationship between the global gamma synchrony and consciousness-related firing in single neurons. Studies that echo the study here, in that they use identical signals that are consciously recognised or not recognised, show with a signal not consciously recognised, there is only local gamma synchrony, but with a conscious signal there is global gamma synchrony between several neuronal assemblies in spatially separated parts of the brain. This could suggest that the conscious response in a single cell is linked to or dependent on global gamma synchrony. However, it would appear not necessary for the whole collection of neuronal assemblies to come into consciousness, but only for the synchrony to trigger consciousness in the individual neuron. This might make it possible to invert Hameroff’s proposal for quantum coherence in neurons to drive consciousness in the gamma synchrony. The opposite case of the synchrony driving consciousness in single neurons would be more compatible with the type of quantum coherence that is functional in photosynthetic organism, but only over femto or picosecond timescales. The authors studied the response of single neurons in the medial temporal lobe, while subjects looked at pictures of familiar faces or landmarks. The response of the neurons studied correlated with conscious perceptions reported by the subjects of the study. Visual perception is processed by the ventral visual pathway, and goes from the primary visual cortex to the medial temporal lobe. Recent studies have shown that neurons in the medial temporal lobe fire selectively to images of individual people. In some trials, the duration of stimuli was right on the boundary of the time needed for recognition of an object, so that it was possible to compare the behaviour of the neurons when an object was recognised and not recognised by the subject. One finding of the study was the ‘all-or-nothing’ nature of the neuronal response. There was no spectrum involved. Either the neuron fired strongly, in correlation with the subject reporting recognition or there was very little activity. The responses were not correlated with the duration of the stimuli, because the responses of the neurons lasted considerable longer than the stimuli. In one study, a single neuron was shown to respond selectively to a picture of the subject’s brother, but not to other people well known to the subject. Particularly noted is the marked difference in the firing of the neuron when the subject’s brother was recognised and not recognised. The stimulus duration of 33 ms meant that half the time the image was recognised, and half the time not recognised. The neuron was nearly silent when the image was not recognised, but fired at nearly 50 Hz when there was conscious recognition, indicating an ‘all-or-nothing’ response from the neuron, linked to subjective report of recognition. The response exceeded the duration of the stimulus, and it was shown that, with a range of durations being used, duration had little influence on the neuron’s response. In another test, a single neuron went from baseline to 10 spikes per second when the subject recognised a picture of the World Trade Centre, but showed little response to all other images that were presented. Again the neuron fired in an ‘all-or-nothing’ fashion, depending on whether there was conscious recognition. In five non-recognition trials this neuron did not fire a single spike. In yet another trial the firing of a single neuron jumped from 0.05 Hz to 50 Hz when the subject reported recognition of an individual. The overall conclusion from these trials is that there is a significant relationship between the firing of neurons in the medial temporal region and the conscious perceptions of subjects. Further to this the activity of the neurons lasted for substantially longer than the stimuli, and had only a marginal correlation with the stimuli. In particular it is noted that with stimuli, at a duration where exactly the same image was recognised in some cases, but not in others, there was an entirely different (all-or-nothing) response from the neuron, according to whether or not the subject consciously recognised the image. Neurons near to the medial temporal neurons studied were shown to respond to different stimuli from the studied neurons. These findings are stated to agree with earlier single-cell studies, including studies involving the inferior temporal cortex and the superior temporal sulcus. |
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