A research project conducted jointly by Milan-Bicocca and San Raffaele Universities has resulted in the first recording of the "voice" of the visual thalamus in a human being, using a non-invasive technique, namely through sensors positioned on the outside of the head. This study sheds light on the role of this structure that is fundamental for our sensory activity, as it sends the brain the images originating from the retina, and reveals how it affects our attention capacity.
The study "Scalp-recorded N40 visual evoked potential: Sensory and attentional properties“ conducted at the laboratory of Cognitive Electrophysiology by Alice Mado Proverbio, psychobiology lecturer at the Milan-Bicocca Department of Psychology, together with Veronica Broido and Francesco De Benedetto, in conjunction with Alberto Zani, psychology lecturer at San Raffaele University, has just been published in the European Journal of Neuroscience. The study introduces a new bioelectric marker of neural activity known as N40, because it is negative and peaks in 40 ms.
In order to record the "voice" of the thalamus, the researchers captured visual evoked potentials (changes in cerebral bioelectric potentials in response to visual input) obtained through 1800 repetitions of stimuli (black and white vertical light bars) in 20 university students wearing a special headset fitted with 128 electrodes.
In the past, this subcortical visual response (N40) had only been detected in monkeys and using invasive neurophysiological techniques (implanting microelectrodes). In humans, electrical responses that can be measured by EEG - a technique that captures electrical signals with sensors attached to the outside of the head - were thought to be limited to those generated by the cortex (the outer surface of the brain) after about 80 ms. This is also due to the particular conformation of the cells present in the thalamus (known as “spiny stellate cells"), which create closed electrical fields and therefore make it difficult to record its activities, unlike those in the visual cortex, which have a linear organisation.
The data collected by the researchers showed that visual evoked potentials (VEPs) can detect signals originating from sub-cortical structures such as the thalamus, before visual information has actually reached the occipital cortex, i.e. in 40 ms, rather than 80 ms as previously thought.
This research has also shown that the thalamus not only transmits sensory signals, but also influences the stimuli to be targeted or ignored, i.e. it acts as an attentive filter, consequently regulating the different level of attention required (e.g.: simply looking or finding a target).
According to the study coordinator, Professor Proverbio, "in the future, it will be important to investigate whether an abnormal visual N40 response can be associated with neural diseases, as is the case for auditory and somato-sensory N40s, which are used as diagnostic criteria for schizophrenia and movement disorders."