Note: the effects of TV on Alpha Brainwaves has been extensively studied, but almost no studies have been done on the effects of TV on Gamma Brainwaves. Why would that be?
"Attention and Brain Activity While Watching Television: Components of Viewer Engagement... Television commercials include elements designed to engage the viewer's attention. Manipulations of the commercial's visual structure that result in rapid pacing or frequent scene changes can be engaging because they require a frequent redirection of visual attention. Manipulations of semantic content through such techniques as the inclusion of humorous or anomalous elements can elicit cognitive engagement. Structural manipulations in videos are known to attenuate the alpha (8 to 13 Hz) rhythm of the electroencephalogram (EEG) recorded over posterior cortical regions involved with visual processes. To examine whether other engaging elements also affect the alpha rhythm, EEG was recorded from 10 participants who viewed television commercials" - Media Psychology (Aug 2004)
"Voluntary Versus Involuntary Alpha Blocking" ... "Abstract: Subjects watched television while EEG was being recorded and later completed a series of recognition tests based on component parts of the commercials they had seen. Memory correlated significantly with changes in the electrical patterns that occurred during viewing. The probability of correct recognition was enhanced when alpha blocking continued for a longer period of time and when hemispheric laterality shifted to the right during the onset of a commercial component and then to the left during the following seconds." - The Journal of Consumer Research (1990)
"As a validity check on the attentional interpretation of alpha, it was shown that mean alpha for each commercial was significantly (negatively) correlated with recall and recognition of commercial contents. The results are discussed in terms of their implications for further use of continuously-recorded alpha in research on factors that influence attention to television." - Pubmed (Aug 1985)
It seems that there are no technical reasons why viewer's brainwaves couldn't relatively easily be measure while watching TV and while engaged in other activities:
"The "gold standard" in the diagnosis of nonepileptic seizures (NESs) is a recording of a typical event during video-EEG monitoring. This procedure is available at all centers specializing in epilepsy and is increasingly available at general hospitals and even in some neurologic group practices. During this procedure, the EEG is recorded for a prolonged period, accompanied by continuous closed-circuit video observation. The digitized EEG and recorded behavior are displayed simultaneously, allowing point-to-point correlations of recorded events and any accompanying electrographic changes." - Epilepsy Professionals (Feb 2004)
EEG Amplitude
"The results indicated that, although EEG amplitude was significantly reduced during TV viewing, there were no consistent changes in evoked response or recovery function measurements." - Journal of Nervous & Mental Disease (Jan 1971)
EEG Diagnosis
"Using EEGs to Diagnose Autism Spectrum Disorders in Infants: Machine-Learning System Finds Differences in Brain Connectivity" - Science Daily (Feb 2011)
"Poor Children's Brain Activity Resembles That Of Stroke Victims, EEG Shows" - Science Daily (Dec 2008)
"Check this video out, Neuroscientist Aditi Shankardass is doing amazing things with EEG technology, especially when it comes to redefining the way we approach diagnosing developmental disorders in children. She argues (compellingly) that looking at a child's brain, not just their behavior, is the key to discovering the truth about ADD and many other challenges" - Psychology Today Blog (Jan 2011)
EEG and Learning
"The Moser experiment is premised on the fact that there are two distinct reactions to mistakes, both of which can be reliably detected using electroenchephalography, or EEG. The first reaction is called error-related negativity (ERN). It appears about 50 milliseconds after a screw-up and is believed to originate in the anterior cingulate cortex, a chunk of tissue that helps monitor behavior, anticipate rewards and regulate attention. This neural reaction is mostly involuntary, the inevitable response to any screw-up." - Wired (Oct 2011)
EEG and the Default Mode
"EEG default mode network in the human brain: spectral regional field powers." - PubMed (Jan 2008)
"Or rather, his brain's performance is near perfect. The man has a machine strapped to his head, an array of electrodes called an electroencephalogram, or EEG, which is recording his brain activity as each image skips by. It then sends the brain-activity data wirelessly to a large computer. The computer has learned what the man's brain activity looks like when he sees one of the visual targets, and, based on that information, it quickly reshuffles the images." - The Chronicle Review (July 2012)
"A concept of brain-rate is introduced, defining it as the weighted mean frequency of the EEG spectrum. In analogue to the blood pressure, heart-rate and temperature, used as standard preliminary indicators of corresponding general bodily activations, it is proposed to use the brain-rate as a preliminary indicator of general mental activation (mental arousal) level. In addition, along with the more specific few-band biofeedback parameters (theta-beta ratio, relative beta ratio, etc.), the brain-rate could be effectively used as a general multiband biofeedback parameter." - PubMed (Dec 2005)
"The Nielsen Co. is to announce today a strategic investment in and alliance with NeuroFocus, which specializes in the practice of measuring brain waves to determine consumers’ responses to marketing messages." - Commercial Alert (Feb 2008)
"EEG default mode network in the human brain: spectral regional field powers." - 
