Prefrontal Modulation of Visual Processing In Humans
This is the transcript of the talk I did in Systems Neuroscience class at UT Dallas.
Disclaimer: The paper being discussed in this post is not my work. You can download the original paper from here.
After reading this paper, I was actually super excited to find myself that this study was only the tip of the iceberg.
Now, since I have got your ATTENTION, let me ask you, what is ATTENTION? ATTENTION, it’s defined as the cognitive and behavioral process of selectively concentrating on a discrete aspect of information. And in this paper, we will be studying visual attention and how various neuronal mechanics are affected by it.
Before going into the details of this paper, it’s is important to know the big picture. What problem is the entire field trying to solve. Understanding visual processing helps us to understand visual attention. And understanding of visual attention when combined with the understanding of motor and auditory attention helps us to better understand working memory, learning and decision making and ultimately the human behavior. And the evidences for the intra-hemispherical control of visual processing, from this study was in accordance with the intra-hemispherical control of auditory and visual processing.
In addition to this, some studies have also demonstrated the plasticity of this region, which is now helping experts to optimize cognitive therapy techniques for rehabilitating patients with prefrontal cortical damage.
Now let’s look at the summaries of some of the researches done before this study.
- Visual attention modifies your sensory inputs to improve your perception
- There is a correlation between attention and changes in connectivity
- Attention affects the sensitivity of neuronal population
Though all these studies were important in understanding visual attention, they failed to consider the temporal dynamics of the prefrontal extrastriate interaction.
And the solution the authors proposed was the inclusion of theory based behavioral testing, along with the physiological recording techniques.
So, what exactly were the authors trying to answer with this specific research? Well, their goal was to find the anatomical, electrophysiological and behavioral evidence for their hypothesis that prefrontal cortex regulates the neuronal activity in extrastriate cortex during visual attention. And they used the temporal resolution of ERPs coupled with lesion analysis to find the evidences to support their hypothesis.
Now let’s look in detail what these researchers did.
For this experiment, 10 patients were selected who had unilateral focal lesion to their dorsolateral prefrontal cortex. Lesion were either due to single stroke or craniotomy. The patients were also matched by 10 controls free of neurological and psychiatric diseases for age, sex and education.
Each of these subjects, so selected, were seated in a chair 1.6m away from the video monitor in a sound attenuated chamber and they were instructed to fixate on a yellow crosshair and to press a button upon detection of randomly occurring targets embodied in streams of task irrelevant stimuli directed to both visual hemi-field. If the subject detected the target within 300-800ms after presenting the target, it was considered a hit or else a miss.
While the subject was performing the task, brain electrical activity was recorded from electrodes placed at these following sites: Fp1, Fpz, Fp2, F7, F3, Fz, F4, F8, T3, C3, Cz, C4, T4, T5, P3, Pz, P4, T6, O1 & O2
Target related mean and peak amplitudes of ERP components were measured relative to a 200ms prestimulus baseline. The mean ERP amplitudes so obtained were subjected to a series of ANOVAs with group (patient, controls) as the between-subject factor and visual hemifield & electrode position as the repeated measures factors.
The researchers also obtained difference waveform by subtracting the standard ERPs from the target ERPs and performed a finer temporal analysis of the attention effects on these difference waves.
From this study, it was observed that controls had a hit rate of 93.9% and patients 82.3%. An interaction between group and field of presentation showed a target detection deficit in the contralessional field in patients.
The reaction time was also seen to be prolonged in the patients.
When the subjects were inattentive, both patients and controls showed the normal pattern of larger early (P1) ERPs over TOc and TOi. Here TOi stands for temporo-ocipital ipsilesion and TOc for temporo-ocipital contralesion.
But for standard stimuli, P1 response over TOi was found to be reduced, however N1 response remained unaffected.
And for target stimuli, N2 components were abolished & P3b response reduced. If you don’t know what a P3b is, the only thing you need to understand is that higher the unexpectedness, lower the P3b response.
So the 2 messages I want you to take away from this post are:
- Prefrontal cortex damage reduces your visual discrimination ability and hence attention
- Studying prefrontal cortex damage compliments the understanding of our current knowledge about information processing in humans and thereby helping to build better models for Computer Vision