Category Archives: PPC

On Working Out The Details

Those interested in figuring out how the brain works have collected data using techniques which probe progressively smaller structures. From observing outward behaviors of the whole animal, to the voltages generated by areas of the brain using EEG, down to the activity of single cells using electrophysiology. The last is the term for any measurement of the electrical responses (more often voltage, but current as well) of individual neurons, and is the sine qua non of nervous system function in that it reflects the millisecond by millisecond goings-on of the brain’s most basic units.

Even at this level, however, there remains an essential ambiguity: while the spiking of the neurons in your eye definitely represents sensory information and the voltage in motor neurons connected to the muscles in your arm reflects motor output, the electrical bustle of units in so called “association areas” of the cerebral cortex, are much more difficult to categorize in such unequivocal fashion. These regions respond to stimulation in many sensory modalities and during motor output, to varying degrees.

The posterior parietal cortex (PPC) is one example which has been extensively examined. If a monkey is given a simple task which connects sensory to motor – say, reaching for or directing one’s gaze towards a target – the neurons in this area will light up. But it is unclear whether their vigor is a response to the stimulus, or is responsible for the invocation of the movement1.

At this point, it may be prudent to ask: couldn’t they be doing both? Yes. However, none of the single-cell electrophysiology experiments aimed at the PPC to-date have been specific enough to discriminate between the possibilities that it is responsible for sensory or motor, or truly a combination of the two.

Enter Richard Andersen, a CalTech researcher who has been working in the field of motor planning for quite some time. His view is in opposition to Columbia’sMickey Goldberg: that the PPC is about attention, not intention.

In Dr. Andersen’s recent work, meant to be the last word in their ongoing debate, and published in Neuron, he uses a new twist on old experiments: allowing the monkey, from whose brain the data are being gathered, to freely decide what type of movement he wants to execute2. The stimuli are always the same, either a red or green ball, and the monkey can choose whether to reach out and touch it, or simply move his gaze towards is (in the reaching case he must keep his gaze elsewhere). The intriguing finding is that there are cells which are selective for the type of movement but not the type of stimulus. It is in this sense which Dr. Andersen thinks he has demonstrated the motoric nature of the PPC.

The beauty of Dr. Andersen’s experiment is that this technique has been around for say 40 years now, and yet we are still able to learn much by savvy applications of its use. Human ingenuity in experimental design has always been the primary drive in scientific discovery, for what good are tools if one doesn’t know how to use them. Don’t get me wrong, technological advancements are essential to scientific progress, but it is simply astounding how simple tweaks on old ideas can open up new avenues of research.


1. Colby, CL & Goldberg, ME (1999) Space and Attention in Parietal Cortex. Annu. Rev. Neurosci. 22:319–349

2. Cui H, & Andersen, RA (2007) Posterior Parietal Cortex Encodes Autonomously Selected Motor Plans. Neuron, V56:552-559