Transient Cortical Excitation at the onset of visual fixation

Visual recognition is brain state dependent

How do we see the world• Scan the environment • Saccades – movement of the eyes from point of

fixation• At each fixation point – a multitude of retinal

outputs enter the system. Question – How does the onset of visual fixation (in

the absence of stimulation) effect the ongoing neural activity of the brain?

Perisaccadic Modulation • Observed in different visual pathway areas ranging from

the LGN to the prefrontal cortex• Suppression followed by enhancement• “priming effect” – for new visual input • Occurs in the absence of visual stimulation

• Pg 10 – “ The brain is perpetually active, even in the absence of environmental and body derived stimuli…in fact a main argument put forward in this book is that most of the brains activity is generated from within and perturbation of this default pattern by external inputs often causes only a minor departure from it’s robust and internally controlled program” – Rhythms of the brain

Fixation-Amplifier Hypothesis

• Developed a hypothesis based on perisaccadic modulation

• Refelective of an underlying mechanism • Increased neural responses to information

entering the retina during the point of fixation

Prove it?

• Developed a study • Animal sat in chair with eyes fixated in a set position • Eye tracker • Complete darkness • LED light• EEG + MUA • V1• 13 experimental sessions • 3 parts to study

Part 1 – CSD and MUA

Describing Fixation quantitatively: AVREC & AVMUA

• Computed grand mean for CSD – AVREC • Computed grand mean for MUA- AVMUA

Mechanisms that bring about excitation

2 possibilities:

1)The effect is local excitation triggered by some internal input – efferent copy of eye muscle command.

2) Phase resetting/phase modulation – which is the reorganization of ongoing activity without the addition of energy to the system

….

• Pg 111 – “ Brain dynamics constantly shift from the complex to the predictable…neural activity shuttles between the interference prone complexity and robust predictable oscillatory synchrony”. – Rhythms of the brain

Assessing Phase Concentration

Continue…

• These findings support the idea that oscillatory phase concentration reflects modulation of the local neural ensemble in preparation for the arrival of visual inputs generated at fixation.

Phase Histogram

2 questions still remain?

• 1)is the phase concentration purely indicative of phase resetting?

• 2) Hilbert Transform which was used to calculate the phase ignores frequency, so how can you interpret these results relative to the frequency bands seen in the EEG?

Amplitude of pre/post fixation in 3 bands:

….• Theta/delta (3-8hz) brings about phase

concentrated synchrony which is needed for optimal responding in the presence of visual fixation.

• Thus – figures 1 and 2 revealed and increase in CSD and MUA following fixation onset and figures 3 and 4 demonstrate that these increases are associated with oscillatory phase concentration and increases in spectral power.

• Do these effects reflect an increase in local cortical excitability?

Ideal vs worst phase

• Wanted to know whether or not the phase distribution during the time of fixation is associated with enhanced excitability?

• Calculated the mean CSD and MUA amplitudes

• Ideal vs worst phase.

Graphic representation of ideal & worst phase

results

• Amplitude is greater in post stimulus• Ideal phase does not reflect excitability rather

onset of fixation does.• Ideal phase is not fundamentally different

from the oscillatory phase

Explanation.

• The idea is that the fixation-related phase resetting of the oscillation can place a new retinal image in an “ideal” optimal excitability phase so that the response is amplified relative to inputs that are NOT synchronized to fixation.

• Makes use of the large amount of energy found in the oscillations

Summary:• 1) Neural modulation at the onset of fixation

reflects an underlying nonvisual mechanism that produces local neural excitation at the point of fixation in V1.

• 2) Fixation onset is associated with significant oscillatory phase concentration

• 3) Synchrony occurred in the “delta/theta” band • 4)Fixation effects reflect the ability of the brain

gaze control system to “prime” or “prepare” the visual system for temporal patterns of visual input.

Implications

• 1) enhances perception • 2) color• 3) Attention• 4) Natural Vision * Refer to last note