Indeed, the son of Pandu, filled with rage, struck off that head which was the root of the evil policy of the Kurus, with that impetuous shaft winged with gold and whetted on stone. Beholding Shakuni lying headless on the ground and all his limbs drenched with gore, thy warriors, rendered powerless with fear, fled away on all sides with weapons in their hands.
The Sanskrit slokhas corresponding to it are
59 sa tacchiro vegavatā śareṇa; suvarṇapuṅkhena śilāśitena prāverayat kupitaḥ pāṇḍuputro; yat tat kurūṇām anayasya mūlam
60 hṛtottamāṅgaṃ śakuniṃ samīkṣya; bhūmau śayānaṃ rudhirārdragātram yodhās tvadīyā bhayanaṣṭa sattvā; diśaḥ prajagmuḥ pragṛhītaśastrāḥ
The bolded portion is interesting to me. The portion is clearly and explicitly stating that the head of Shakuni was the root of the evil policy of the Kurus. It is well known that Shakuni has capability of controlling dies and he has bad behavior from childhood.
From the statement it can be inferred that head has the capability of thinking. Since the implicit meaning of the portion is the thoughts of Shakuni is reason for evil policies of Kurus.
So it can be inferred that head as a part of human body that has capability of thinking is known to people during Mahabharata times (atleast to Vyasa).
In this context my doubt is: Are there any Hindu scriptures earlier(historically) to Mahabharata contains either implicitly or explicitly the fact head has the capability of thinking.
Note: Since I dont know the actual Sanskrit words, I am analyzing with the above translation only.