ACTION-BLINDSIGHT IN HEALTHY SUBJECTS AFTER TRANSCRANIAL MAGNETIC - - PowerPoint PPT Presentation

ACTION-BLINDSIGHT IN HEALTHY SUBJECTS AFTER TRANSCRANIAL MAGNETIC STIMULATION

Review of Christensen, Kristiansen, Rowe & Nielsen (2008).

Louis Janse van Rensburg

Background to Study

¨ Blindsight: Ability to respond to visual information without conscious perception of

visual stimuli (Sanders et al., 1974).

¨ Efference copy and movement correction: ¤ Proposed mechanisms of blindsight: n Subcortical: involving brainstem nuclei & propriospinal neurons located in the cervical spine

(Alstermark et al., 1990, 1999, 2007 and Pierrot-Deselligny et al., 2004)

n Fast reaching movements in cats, monkeys and humans (faster than possible for implication of trans-

cortical regions)

n Alternative cortical paths: Visual Cortex bypassed, SCà Pulvinar à Parietal Motor Regions

(Weizkrantz, 2002)

¨ Prior to Christensen et al., literature only demonstrates lesion cases: ¤ Problematic due to confounds: injury/post injury neuroplasticity ¤ TMS induces blindsight in healthy subjects and avoids above confounds. ¨ Rejected dichotomy of perception/no perception, ¤ Used subjective perception awareness scale (Overgaard et al 2006). ¤ Allows for testing of interaction of perceptual clarity and performance (i.e. impairment due

to parallel distribution of resources).

Background to Study (2)

¨ Cotterill (2001) postulates sensory-perception and

motor control linked:

¤ Covert movement “simulation” of interacting with

stimulus that leads to perception of that stimulus

n i.e. “event coding” (Hommel et al., 2001) ¤ Motor region correlates to subjective visual

experience of objects (Reis et al., 2002, and Christensen et al., 2006):

¤ Intra-parietal cortex ¤ Premotor cortex

¨ Postulated Binding of Perception & Action (Gotlieb,

2007):

¤ LIP

QUESTION: what does this mean for general perception? Is every object perceived dependent on it’s “utility” and if so what is the extent of this simulated “interacting with” the stimulus?

Aim and Hypotheses

¨ Aim: To induce blindsight through TMS and investigate the interaction

between sensory perception and motor control

¨ Hypotheses:

1.

TMS would decrease/inhibit ability to consciously percieve objects

n DVs = i) report on test target’s arrow direction as well as ii) clarity (C1= no perception)

2.

TMS (with perception reports at level C1) would enhance correction movement responding when required in second light condition vs no second light condition (error).

n

DV = fraction of movement corrections without perception

3.

Interaction between perceptual clarity and motor performance such that greater perceptual clarity would inhibit motor performance.

n

i.e. increase in CRT or RT with increase in clarity

n

Reason for this is “networks responsible for perception/action… responsible for either perception or action” Simultaneous processing is not optimal.

4.

CRT will be lower than RT (I.e. due to efference copy during TMS and movement correction)

Materials and Methods

¨ Subjects

¤ 11healthy adults

n Range in years 20-44 n Mean age 28.1 years n 2 subjects omitted;

1.

Subject 10- Phosphenes reported after 19th trial

2.

Subject 11- No TMS effect on ability to report shape (no comparative inhibition of conscious visual perception)

¨ Setup

¤ SB placed in front of RH (Subjects placed RH on SB). ¤ MB placed 52cm in front of SB ¤ LB & RB placed 9.5 cm to left/right of MB ¤ LED panels placed above targets ¤ Infrared camera & reflective sticker (placed on

middle finger) to trace movements

¤ Synchronized camera with trigger pulses, LED

presentations and TMS (Qualisys analog sampling board)

¨ TMS Stimulation (Inhibition)

¤ Magstim 200 ¤ Single pulse,100% intensity or less (80%-90%) if

“uncomfortable”

¤ Circular coil 13.5 outer diameter and 5cm inner

diameter

Materials and Methods

Materials and Methods

¨

Task and Visual Presentation

¤

200 (randomized) trials per subject

¤

Pre-trial, RH placed on SB

¤

MP LED presented above MB

¤

In approx. 80 trials (condition A), 7ms after release from SB and movement to MB, LP/RP LEDs presented (approx. 40 trials each).

¤

Second LED presentation took form of arrows. 20-800ms duration to control for attentional drift.

¤

TMS/Sham TMS (equal across trials) administered over visual cortex 80-90 seconds after second light in correction-condition, or, 87-97 seconds after release of SB as in no correction condition.

7 80-90

Pre-trial RH on SB

80/200 trials 120/200 trials

Pre-trial RH on SB

87-97

Materials and Methods

¨ Post trial verbal report: 1.

Whether subject had performed corrective movement to RB/LB or not.

2.

Which arrow they had perceived on LP or RP.

3.

Reported clarity of stimulus on modified perception awareness graded scale (Overgaard et al 2006) : C1(no perception) – C5 (“definite perception”).

Results

Hypothesis 1: TMS would decrease/inhibit ability to consciously perceive objects

¨ Found TMS significantly decreased ability to identify

arrow direction

Result

Hypothesis 2: TMS (with perception reports at level C1) would enhance correction movement responding when required in second light condition vs no second light condition (error).

¨ Found: “correction” movements

significantly more frequent in light vs no light (false positive movements)

Results

Hypothesis 3: Interaction between perceptual clarity and motor performance such that greater perceptual clarity would inhibit motor performance.

¨ Found: Null retained, no significant interaction.

Results

Hypothesis 4: CRT will be lower than RT (i.e. Efference copy)

¤

Found: CRT was significantly lower than RT

n

At C1 level, TMS applied and Correction valid (i.e. reaches target)

¨ Control Measure:

¤ MT (total movement time) may influence ability to perform corrective movement

(DV = fraction of correct corrections).

n i.e. movement itself may aid correction movement (efferent based)

¤ Found: MT, Clarity and interaction between MT and Clarity do not explain

ability to perform correction movements (in favor of efference copy account).

Discussion

¨ TMS decreases visual perception ¨ Despite loss of perception (lowest level = C1), TMS

does not inhibit corrective movement.

¤ TMS eliminates existing confounds to blindsight (I.e. broader

injury, neuroplasticity post injury

¨ Evidence for Efference copy in online movement

correction (CRT < RT).

¤ Reason:

n Initial RT requires visual signal processed via visual cortexà

Motor cortex

n Efference copy advantage: deviation of performed from intended

movement is quickly adjusted.

Discussion (2)

¨ Bypassing of Visual Cortex:

¤ Subcortical:

n Acallosal Patient is as fast as healthy subjects in fast reaching task

(similar to current study) (Day & Brown, 2001). Subcortical mechanisms deemed necessary for corrective movements.

n Cats perform fast movements in similar study, (Alstermark et al.,

1990). Involvement of networks that bypass visual cortex completely.

n Propriospinal neurons in cervical spine n Retino-tectospinal and retino-tecto-reticularspinal pathways n Brainstem

¤ Alternative subcortical route (Humans):

n SCà Pulvinarà Parietal Cortex

Discussion (3)

¨ Did not find hypothesized interaction between

perception and action.

¤ Parallel? Or Resource/structurally limited but not

isolated in this study

¨ Possible confounds:

¤ CRT, although significant, not as low as found in other

studies: (Day & Brown, 2001).

n Definitional and task differences (i.e. length of required

movements, and size of targets)

n OR, correction during movement is aided by movement. They

do not explore this further.

Conclusion

¨ TMS induces blindsight ¨ Efference copy involved in motor corrections ¨ But unexpectedly, no interaction between graded

degree of perception and correction performance.