To quantify the disparity between responses to acceleration and deceleration we calculated an Acceleration-Deceleration Tuning Index. Cells with ADTI > 0 have larger responses to acceleration than deceleration. This was the case in nearly all cells, at all ramp rates, as indicated by the population averages above. Further, the ADTI increased with ramp rate.

Adaptation and speed tuning predict acceleration responses in cat cortical areas 17, 18 and PMLS

Nicholas Price1, Nathan Crowder, Markus Hietanen, Michael IbbotsonVisual Sciences, RSBS, Australian National University, Canberra, ACT

1Current address: Neurobiology, Harvard Medical School, Boston MA - nicholas_price@hms.harvard.edu

619.2

MethodsWe recorded the responses of 146 neurons in V1, V2

and PMLS of 9 anaesthetised cats using stimuli confined to each cell’s classical receptive field and moving in the preferred direction.

Two stimuli were used: random texture (0.8° elements) and a 1D smoothed random noise stimulus.

Conclusions

Studies of motion-sensitive neurons traditionally use fixed-speed stimuli. However, in natural viewing, speeds continually change, thus acceleration is a fundamental aspect of vision.

Visual acceleration sensitivity has been identified in the brainstem and is thought to play a role in driving pursuit or optokinetic eye movements.

We have previously shown that acceleration tuning is not a characteristic feature of individual neurons in area MT of the awake macaque.

Here we study the responses of neurons in cat cortical areas 17/V1, 18/V2 and the posteromedial lateral suprasylvian area (PMLS) to stimuli with constant speeds (speed steps) and linearly changing speed (speed ramps comprising acceleration and deceleration).

We assessed:

- The prevalance of speed and acceleration tuning - If a neuron’s preferred speed can be inferred from its responses to speed ramps- if a neuron’s response to acceleration can be predicted from the neuron’s speed tuning and adaptation properties

3. Speed and acceleration tuning

4. Measuring preferred speed Initially, the responses to ramp stimuli were predicted directly from the speed tuning curves, with the predicted response at each point in time determined from the transient and sustained speed tuning curves. This gives the upper and lower boundaries of the red shaded regions in the plots below.

To incorporate the effects of adaptation, we defined a spiking-rate dependent adaptation state ‘At’:

At+δt = At.e(-δt/τ) if Rt ≥ Rthresh

At+δt = 1 + (At-1).e(-δt/ τ) if Rt < Rthresh

The responses at each point in time were then predicted:Rt + t,Late = At . Rtrans(st) + (1-At) . Rsust(st)

- Rt is the predicted response at time t; - Rthresh is a threshold response that governs whether the

cell sensitivity recovers or adapts; - tLate is the cell’s latency at its preferred speed; - st is the stimulus speed at time t; - Rtrans and Rsust are the measured transient and sustained

responses to the specified speed.The time constant “τ” of the change in sensitivity was

determined both from fits to the responses to constant speeds and by setting it as a free parameter and minimising the error between the predicted and measured acceleration responses.

5. Acceleration vs Deceleration

Tuning of responses for speed or acceleration was assessed using:

1.The quality of skewed-Gaussian fits (R2)

2.The presence of systematic response variation, with a clear preferred speed or acceleration.

The majority of cells (74%) in all areas were speed tuned.Only 11% of cells showed clear tuning for acceleration or deceleration.

A cell’s preferred speed was determined in 4 ways:

- The speed steps generating the highest transient (SpPref, Trans) and sustained (SpPref, Sust) responses.

- The stimulus speed at the time of the peak response to acceleration (SpPref, Acc) and deceleration (SpPref, Dec).

None of these measures preferred speed were highly correlated.

However, across the population:

SpPref, Trans > SPref, Sust

SpPref, Dec > SPref Acc > SPref Sust

DecAcc

DecAcc

RR

RRADTI

Related publicationsPrice NSC, Crowder NA, Hietanen MA, Ibbotson MR (2005) Neurons in V1, V2 and

PMLS of cat cortex are speed tuned but not accelerated tuned: the influence of motion adaptation. J Neurophysiology, in press.

Price NSC, Ono S, Mustari MJ, Ibbotson MR (2005) Comparing acceleration and speed tuning in macaque MT: physiology and modeling. J Neurophysiol. 94(5):3451-64

1. Responses to speed stepsSpeed sensitivity was tested with constant speed

steps of 2-240°/s lasting 3 s.Transient responses were calculated from the peak

firing rate within a sliding 24 ms during the first 200 ms of motion.

Sustained responses were averaged from the response 0.5-3 s after motion onset.

Speed tuning curves show the best fit skewed Gaussian.

2. Responses to acceleration

Acceleration sensitivity was tested with speed ramps of 30-2400°/s2 up to a plateau speed of 120 or 240°/s.

Transient responses to acceleration and deceleration were calculated from the peak firing rate within a sliding 24 ms throughout the ramp period.

Sustained responses were averaged from the response to the entire ramp period.

Speed tuning, but not acceleration tuning, is a characteristic feature of neurons in areas V1, V2 and PMLS of the anaesthetised cat.

The prevalance of speed tuning is essentially invariant across the three regions.

There is poor correlation between the preferred speed of a neuron assessed using the responses to steps in stimulus speed and inferred from the responses to acceleration and deceleration.

The temporal profile a neuron’s response to speed ramps cannot be adequately captured by the cell’s transient or sustained speed tuning curves alone.

Incorporating response-dependent adaptation improves the ability to predict a neuron’s responses to accelerating stimuli.

6. Predicting ramp responses

2°